I'm also affected by this bug. It doesn't happen every time on restart, but often.

Below is from my Lightdm.log. Hopefully it will be helpful in resolving this very annoying bug. It also happens when logging in/out from one user to another.

"[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.00s] DEBUG: Starting Light Display Manager 1.1.9, UID=0 PID=1167
[+0.00s] DEBUG: Loaded configuration from /etc/lightdm/lightdm.conf
[+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.00s] DEBUG: Registered seat module xlocal
[+0.00s] DEBUG: Registered seat module xremote
[+0.00s] DEBUG: Adding default seat
[+0.00s] DEBUG: Starting seat
[+0.00s] DEBUG: Starting new display for greeter
[+0.00s] DEBUG: Starting local X display
[+0.02s] DEBUG: X server :0 will replace Plymouth
[+0.05s] DEBUG: Using VT 7
[+0.05s] DEBUG: Activating VT 7
[+0.05s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+0.05s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+0.05s] DEBUG: Launching X Server
[+0.05s] DEBUG: Launching process 1204: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch -background none
[+0.05s] DEBUG: Waiting for ready signal from X server :0
[+0.05s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.05s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.85s] DEBUG: Got signal 10 from process 1204
[+0.85s] DEBUG: Got signal from X server :0
[+0.85s] DEBUG: Stopping Plymouth, X server is ready
[+0.88s] DEBUG: Connecting to XServer :0
[+0.88s] DEBUG: Starting greeter
[+0.88s] DEBUG: Started session 1403 with service 'lightdm', username 'lightdm'
[+0.96s] DEBUG: Session 1403 authentication complete with return value 0: Success
[+0.96s] DEBUG: Greeter authorized
[+0.96s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+0.96s] DEBUG: Session 1403 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+1.21s] DEBUG: Greeter connected version=1.1.9
[+1.21s] DEBUG: Greeter connected, display is ready
[+1.21s] DEBUG: New display ready, switching to it
[+1.21s] DEBUG: Activating VT 7
[+1.97s] DEBUG: Greeter start authentication for baldrick
[+1.97s] DEBUG: Started session 1650 with service 'lightdm', username 'baldrick'
[+1.98s] DEBUG: Session 1650 got 1 message(s) from PAM
[+1.98s] DEBUG: Prompt greeter with 1 message(s)
[+4.13s] DEBUG: Session 1403 exited with return value 0
[+4.14s] DEBUG: Greeter quit
[+4.14s] DEBUG: Failed to start greeter
[+4.14s] DEBUG: Stopping display
[+4.14s] DEBUG: Sending signal 15 to process 1204
[+4.15s] DEBUG: Greeter closed communication channel
[+4.24s] DEBUG: Process 1204 exited with return value 0
[+4.24s] DEBUG: X server stopped
[+4.24s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+4.24s] DEBUG: Releasing VT 7
[+4.24s] DEBUG: Display server stopped
[+4.24s] DEBUG: Display stopped
[+4.24s] DEBUG: Active display stopped, switching to greeter
[+4.24s] DEBUG: Switching to greeter
[+4.24s] DEBUG: Starting new display for greeter
[+4.24s] DEBUG: Starting local X display
[+4.24s] DEBUG: Using VT 7
[+4.24s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+4.24s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+4.24s] DEBUG: Launching X Server
[+4.24s] DEBUG: Launching process 1778: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+4.24s] DEBUG: Waiting for ready signal from X server :0
[+5.11s] DEBUG: Got signal 10 from process 1778
[+5.11s] DEBUG: Got signal from X server :0
[+5.11s] DEBUG: Connecting to XServer :0
[+5.11s] DEBUG: Starting greeter
[+5.11s] DEBUG: Started session 1782 with service 'lightdm', username 'lightdm'
[+5.13s] DEBUG: Session 1782 authentication complete with return value 0: Success
[+5.13s] DEBUG: Greeter authorized
[+5.13s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+5.13s] DEBUG: Session 1782 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+5.32s] DEBUG: Greeter connected version=1.1.9
[+5.32s] DEBUG: Greeter connected, display is ready
[+5.32s] DEBUG: New display ready, switching to it
[+5.32s] DEBUG: Activating VT 7
[+5.32s] DEBUG: Stopping greeter display being switched from
[+6.33s] DEBUG: Greeter start authentication for baldrick
[+6.33s] DEBUG: Started session 1872 with service 'lightdm', username 'baldrick'
[+6.34s] DEBUG: Session 1872 got 1 message(s) from PAM
[+6.34s] DEBUG: Prompt greeter with 1 message(s)
[+7.05s] DEBUG: Session 1782 exited with return value 0
[+7.05s] DEBUG: Greeter quit
[+7.05s] DEBUG: Failed to start greeter
[+7.05s] DEBUG: Stopping display
[+7.05s] DEBUG: Sending signal 15 to process 1778
[+7.06s] DEBUG: Greeter closed communication channel
[+7.19s] DEBUG: Process 1778 exited with return value 0
[+7.19s] DEBUG: X server stopped
[+7.19s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+7.19s] DEBUG: Releasing VT 7
[+7.19s] DEBUG: Display server stopped
[+7.19s] DEBUG: Display stopped
[+7.19s] DEBUG: Active display stopped, switching to greeter
[+7.19s] DEBUG: Switching to greeter
[+7.19s] DEBUG: Starting new display for greeter
[+7.19s] DEBUG: Starting local X display
[+7.19s] DEBUG: Using VT 7
[+7.19s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+7.19s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+7.20s] DEBUG: Launching X Server
[+7.20s] DEBUG: Launching process 1920: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+7.20s] DEBUG: Waiting for ready signal from X server :0
[+7.87s] DEBUG: Got signal 10 from process 1920
[+7.87s] DEBUG: Got signal from X server :0
[+7.87s] DEBUG: Connecting to XServer :0
[+7.87s] DEBUG: Starting greeter
[+7.87s] DEBUG: Started session 1924 with service 'lightdm', username 'lightdm'
[+7.90s] DEBUG: Session 1924 authentication complete with return value 0: Success
[+7.90s] DEBUG: Greeter authorized
[+7.90s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+7.90s] DEBUG: Session 1924 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+8.15s] DEBUG: Greeter connected version=1.1.9
[+8.15s] DEBUG: Greeter connected, display is ready
[+8.15s] DEBUG: New display ready, switching to it
[+8.15s] DEBUG: Activating VT 7
[+8.15s] DEBUG: Stopping greeter display being switched from
[+9.01s] DEBUG: Greeter start authentication for baldrick
[+9.01s] DEBUG: Started session 2010 with service 'lightdm', username 'baldrick'
[+9.02s] DEBUG: Session 2010 got 1 message(s) from PAM
[+9.02s] DEBUG: Prompt greeter with 1 message(s)
[+9.63s] DEBUG: Session 1924 exited with return value 0
[+9.63s] DEBUG: Greeter quit
[+9.63s] DEBUG: Failed to start greeter
[+9.63s] DEBUG: Stopping display
[+9.63s] DEBUG: Sending signal 15 to process 1920
[+9.75s] DEBUG: Process 1920 exited with return value 0
[+9.75s] DEBUG: X server stopped
[+9.75s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+9.75s] DEBUG: Releasing VT 7
[+9.75s] DEBUG: Display server stopped
[+9.75s] DEBUG: Display stopped
[+9.75s] DEBUG: Active display stopped, switching to greeter
[+9.75s] DEBUG: Switching to greeter
[+9.75s] DEBUG: Starting new display for greeter
[+9.75s] DEBUG: Starting local X display
[+9.75s] DEBUG: Using VT 7
[+9.75s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+9.75s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+9.75s] DEBUG: Launching X Server
[+9.75s] DEBUG: Launching process 2058: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+9.75s] DEBUG: Waiting for ready signal from X server :0
[+10.38s] DEBUG: Greeter closed communication channel
[+10.49s] DEBUG: Got signal 10 from process 2058
[+10.49s] DEBUG: Got signal from X server :0
[+10.49s] DEBUG: Connecting to XServer :0
[+10.49s] DEBUG: Starting greeter
[+10.49s] DEBUG: Started session 2134 with service 'lightdm', username 'lightdm'
[+10.52s] DEBUG: Session 2134 authentication complete with return value 0: Success
[+10.52s] DEBUG: Greeter authorized
[+10.52s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+10.52s] DEBUG: Session 2134 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+10.70s] DEBUG: Greeter connected version=1.1.9
[+10.70s] DEBUG: Greeter connected, display is ready
[+10.70s] DEBUG: New display ready, switching to it
[+10.70s] DEBUG: Activating VT 7
[+10.70s] DEBUG: Stopping greeter display being switched from
[+11.56s] DEBUG: Greeter start authentication for baldrick
[+11.56s] DEBUG: Started session 2221 with service 'lightdm', username 'baldrick'
[+11.58s] DEBUG: Session 2221 got 1 message(s) from PAM
[+11.58s] DEBUG: Prompt greeter with 1 message(s)
[+12.24s] DEBUG: Session 2134 exited with return value 0
[+12.24s] DEBUG: Greeter quit
[+12.24s] DEBUG: Failed to start greeter
[+12.24s] DEBUG: Stopping display
[+12.24s] DEBUG: Sending signal 15 to process 2058
[+12.29s] DEBUG: Greeter closed communication channel
[+12.40s] DEBUG: Process 2058 exited with return value 0
[+12.40s] DEBUG: X server stopped
[+12.40s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+12.40s] DEBUG: Releasing VT 7
[+12.40s] DEBUG: Display server stopped
[+12.40s] DEBUG: Display stopped
[+12.40s] DEBUG: Active display stopped, switching to greeter
[+12.40s] DEBUG: Switching to greeter
[+12.40s] DEBUG: Starting new display for greeter
[+12.40s] DEBUG: Starting local X display
[+12.40s] DEBUG: Using VT 7
[+12.40s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+12.40s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+12.40s] DEBUG: Launching X Server
[+12.40s] DEBUG: Launching process 2271: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+12.40s] DEBUG: Waiting for ready signal from X server :0
[+13.08s] DEBUG: Got signal 10 from process 2271
[+13.09s] DEBUG: Got signal from X server :0
[+13.09s] DEBUG: Connecting to XServer :0
[+13.09s] DEBUG: Starting greeter
[+13.09s] DEBUG: Started session 2275 with service 'lightdm', username 'lightdm'
[+13.11s] DEBUG: Session 2275 authentication complete with return value 0: Success
[+13.11s] DEBUG: Greeter authorized
[+13.11s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+13.12s] DEBUG: Session 2275 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+13.31s] DEBUG: Greeter connected version=1.1.9
[+13.31s] DEBUG: Greeter connected, display is ready
[+13.31s] DEBUG: New display ready, switching to it
[+13.31s] DEBUG: Activating VT 7
[+13.31s] DEBUG: Stopping greeter display being switched from
[+14.14s] DEBUG: Greeter start authentication for baldrick
[+14.14s] DEBUG: Started session 2362 with service 'lightdm', username 'baldrick'
[+14.15s] DEBUG: Session 2362 got 1 message(s) from PAM
[+14.15s] DEBUG: Prompt greeter with 1 message(s)
[+14.83s] DEBUG: Session 2275 exited with return value 0
[+14.83s] DEBUG: Greeter quit
[+14.83s] DEBUG: Failed to start greeter
[+14.83s] DEBUG: Stopping display
[+14.83s] DEBUG: Sending signal 15 to process 2271
[+14.88s] DEBUG: Greeter closed communication channel
[+14.98s] DEBUG: Process 2271 exited with return value 0
[+14.98s] DEBUG: X server stopped
[+14.98s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+14.98s] DEBUG: Releasing VT 7
[+14.98s] DEBUG: Display server stopped
[+14.98s] DEBUG: Display stopped
[+14.98s] DEBUG: Active display stopped, switching to greeter
[+14.98s] DEBUG: Switching to greeter
[+14.98s] DEBUG: Starting new display for greeter
[+14.98s] DEBUG: Starting local X display
[+14.98s] DEBUG: Using VT 7
[+14.98s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+14.98s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+14.98s] DEBUG: Launching X Server
[+14.98s] DEBUG: Launching process 2412: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+14.98s] DEBUG: Waiting for ready signal from X server :0
[+15.67s] DEBUG: Got signal 10 from process 2412
[+15.67s] DEBUG: Got signal from X server :0
[+15.67s] DEBUG: Connecting to XServer :0
[+15.67s] DEBUG: Starting greeter
[+15.67s] DEBUG: Started session 2416 with service 'lightdm', username 'lightdm'
[+15.70s] DEBUG: Session 2416 authentication complete with return value 0: Success
[+15.70s] DEBUG: Greeter authorized
[+15.70s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+15.70s] DEBUG: Session 2416 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+15.89s] DEBUG: Greeter connected version=1.1.9
[+15.89s] DEBUG: Greeter connected, display is ready
[+15.89s] DEBUG: New display ready, switching to it
[+15.89s] DEBUG: Activating VT 7
[+15.89s] DEBUG: Stopping greeter display being switched from
[+16.73s] DEBUG: Greeter start authentication for baldrick
[+16.73s] DEBUG: Started session 2502 with service 'lightdm', username 'baldrick'
[+16.74s] DEBUG: Session 2502 got 1 message(s) from PAM
[+16.74s] DEBUG: Prompt greeter with 1 message(s)
[+17.40s] DEBUG: Session 2416 exited with return value 0
[+17.40s] DEBUG: Greeter quit
[+17.40s] DEBUG: Failed to start greeter
[+17.40s] DEBUG: Stopping display
[+17.40s] DEBUG: Sending signal 15 to process 2412
[+17.45s] DEBUG: Greeter closed communication channel
[+17.55s] DEBUG: Process 2412 exited with return value 0
[+17.55s] DEBUG: X server stopped
[+17.55s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+17.55s] DEBUG: Releasing VT 7
[+17.55s] DEBUG: Display server stopped
[+17.55s] DEBUG: Display stopped
[+17.55s] DEBUG: Active display stopped, switching to greeter
[+17.55s] DEBUG: Switching to greeter
[+17.55s] DEBUG: Starting new display for greeter
[+17.55s] DEBUG: Starting local X display
[+17.55s] DEBUG: Using VT 7
[+17.55s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+17.55s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+17.55s] DEBUG: Launching X Server
[+17.55s] DEBUG: Launching process 2552: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+17.55s] DEBUG: Waiting for ready signal from X server :0
[+18.25s] DEBUG: Got signal 10 from process 2552
[+18.25s] DEBUG: Got signal from X server :0
[+18.25s] DEBUG: Connecting to XServer :0
[+18.25s] DEBUG: Starting greeter
[+18.25s] DEBUG: Started session 2556 with service 'lightdm', username 'lightdm'
[+18.28s] DEBUG: Session 2556 authentication complete with return value 0: Success
[+18.28s] DEBUG: Greeter authorized
[+18.28s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+18.28s] DEBUG: Session 2556 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+18.46s] DEBUG: Greeter connected version=1.1.9
[+18.46s] DEBUG: Greeter connected, display is ready
[+18.46s] DEBUG: New display ready, switching to it
[+18.46s] DEBUG: Activating VT 7
[+18.46s] DEBUG: Stopping greeter display being switched from
[+19.29s] DEBUG: Greeter start authentication for baldrick
[+19.29s] DEBUG: Started session 2644 with service 'lightdm', username 'baldrick'
[+19.30s] DEBUG: Session 2644 got 1 message(s) from PAM
[+19.30s] DEBUG: Prompt greeter with 1 message(s)
[+19.94s] DEBUG: Session 2556 exited with return value 0
[+19.94s] DEBUG: Greeter quit
[+19.94s] DEBUG: Failed to start greeter
[+19.94s] DEBUG: Stopping display
[+19.94s] DEBUG: Sending signal 15 to process 2552
[+20.00s] DEBUG: Greeter closed communication channel
[+20.10s] DEBUG: Process 2552 exited with return value 0
[+20.10s] DEBUG: X server stopped
[+20.10s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+20.10s] DEBUG: Releasing VT 7
[+20.10s] DEBUG: Display server stopped
[+20.10s] DEBUG: Display stopped
[+20.10s] DEBUG: Active display stopped, switching to greeter
[+20.10s] DEBUG: Switching to greeter
[+20.10s] DEBUG: Starting new display for greeter
[+20.10s] DEBUG: Starting local X display
[+20.10s] DEBUG: Using VT 7
[+20.10s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+20.10s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+20.10s] DEBUG: Launching X Server
[+20.10s] DEBUG: Launching process 2693: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+20.10s] DEBUG: Waiting for ready signal from X server :0
[+20.80s] DEBUG: Got signal 10 from process 2693
[+20.80s] DEBUG: Got signal from X server :0
[+20.80s] DEBUG: Connecting to XServer :0
[+20.80s] DEBUG: Starting greeter
[+20.80s] DEBUG: Started session 2697 with service 'lightdm', username 'lightdm'
[+20.83s] DEBUG: Session 2697 authentication complete with return value 0: Success
[+20.83s] DEBUG: Greeter authorized
[+20.83s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+20.83s] DEBUG: Session 2697 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+21.01s] DEBUG: Greeter connected version=1.1.9
[+21.01s] DEBUG: Greeter connected, display is ready
[+21.01s] DEBUG: New display ready, switching to it
[+21.01s] DEBUG: Activating VT 7
[+21.01s] DEBUG: Stopping greeter display being switched from
[+21.86s] DEBUG: Greeter start authentication for baldrick
[+21.86s] DEBUG: Started session 2784 with service 'lightdm', username 'baldrick'
[+21.87s] DEBUG: Session 2784 got 1 message(s) from PAM
[+21.87s] DEBUG: Prompt greeter with 1 message(s)
[+22.52s] DEBUG: Session 2697 exited with return value 0
[+22.52s] DEBUG: Greeter quit
[+22.52s] DEBUG: Failed to start greeter
[+22.52s] DEBUG: Stopping display
[+22.52s] DEBUG: Sending signal 15 to process 2693
[+22.60s] DEBUG: Greeter closed communication channel
[+22.70s] DEBUG: Process 2693 exited with return value 0
[+22.70s] DEBUG: X server stopped
[+22.70s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+22.70s] DEBUG: Releasing VT 7
[+22.70s] DEBUG: Display server stopped
[+22.70s] DEBUG: Display stopped
[+22.70s] DEBUG: Active display stopped, switching to greeter
[+22.70s] DEBUG: Switching to greeter
[+22.70s] DEBUG: Starting new display for greeter
[+22.70s] DEBUG: Starting local X display
[+22.70s] DEBUG: Using VT 7
[+22.70s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+22.70s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+22.70s] DEBUG: Launching X Server
[+22.70s] DEBUG: Launching process 2833: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+22.70s] DEBUG: Waiting for ready signal from X server :0
[+23.39s] DEBUG: Got signal 10 from process 2833
[+23.39s] DEBUG: Got signal from X server :0
[+23.39s] DEBUG: Connecting to XServer :0
[+23.39s] DEBUG: Starting greeter
[+23.39s] DEBUG: Started session 2837 with service 'lightdm', username 'lightdm'
[+23.42s] DEBUG: Session 2837 authentication complete with return value 0: Success
[+23.42s] DEBUG: Greeter authorized
[+23.42s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+23.42s] DEBUG: Session 2837 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+23.60s] DEBUG: Greeter connected version=1.1.9
[+23.60s] DEBUG: Greeter connected, display is ready
[+23.60s] DEBUG: New display ready, switching to it
[+23.60s] DEBUG: Activating VT 7
[+23.60s] DEBUG: Stopping greeter display being switched from
[+24.45s] DEBUG: Greeter start authentication for baldrick
[+24.45s] DEBUG: Started session 2924 with service 'lightdm', username 'baldrick'
[+24.46s] DEBUG: Session 2924 got 1 message(s) from PAM
[+24.46s] DEBUG: Prompt greeter with 1 message(s)
[+25.10s] DEBUG: Session 2837 exited with return value 0
[+25.10s] DEBUG: Greeter quit
[+25.10s] DEBUG: Failed to start greeter
[+25.10s] DEBUG: Stopping display
[+25.10s] DEBUG: Sending signal 15 to process 2833
[+25.11s] DEBUG: Greeter closed communication channel
[+25.22s] DEBUG: Process 2833 exited with return value 0
[+25.22s] DEBUG: X server stopped
[+25.22s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+25.22s] DEBUG: Releasing VT 7
[+25.22s] DEBUG: Display server stopped
[+25.22s] DEBUG: Display stopped
[+25.22s] DEBUG: Active display stopped, switching to greeter
[+25.22s] DEBUG: Switching to greeter
[+25.22s] DEBUG: Starting new display for greeter
[+25.22s] DEBUG: Starting local X display
[+25.22s] DEBUG: Using VT 7
[+25.22s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+25.22s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+25.22s] DEBUG: Launching X Server
[+25.22s] DEBUG: Launching process 2972: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+25.22s] DEBUG: Waiting for ready signal from X server :0
[+25.91s] DEBUG: Got signal 10 from process 2972
[+25.91s] DEBUG: Got signal from X server :0
[+25.91s] DEBUG: Connecting to XServer :0
[+25.91s] DEBUG: Starting greeter
[+25.91s] DEBUG: Started session 2976 with service 'lightdm', username 'lightdm'
[+25.94s] DEBUG: Session 2976 authentication complete with return value 0: Success
[+25.94s] DEBUG: Greeter authorized
[+25.94s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+25.94s] DEBUG: Session 2976 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+26.12s] DEBUG: Greeter connected version=1.1.9
[+26.12s] DEBUG: Greeter connected, display is ready
[+26.12s] DEBUG: New display ready, switching to it
[+26.12s] DEBUG: Activating VT 7
[+26.12s] DEBUG: Stopping greeter display being switched from
[+26.99s] DEBUG: Greeter start authentication for baldrick
[+26.99s] DEBUG: Started session 3064 with service 'lightdm', username 'baldrick'
[+27.00s] DEBUG: Session 3064 got 1 message(s) from PAM
[+27.00s] DEBUG: Prompt greeter with 1 message(s)
[+33.69s] DEBUG: Continue authentication
[+33.76s] DEBUG: Session 3064 authentication complete with return value 0: Success
[+33.76s] DEBUG: Authenticate result for user baldrick: Success
[+33.78s] DEBUG: User baldrick authorized
[+33.78s] DEBUG: Greeter requests session ubuntu
[+33.78s] DEBUG: Using session ubuntu
[+33.78s] DEBUG: Stopping greeter
[+33.78s] DEBUG: Session 2976: Sending SIGTERM
[+33.83s] DEBUG: Session 2976 exited with return value 0
[+33.83s] DEBUG: Greeter quit
[+33.86s] DEBUG: Dropping privileges to uid 1000
[+33.86s] DEBUG: Restoring privileges
[+33.87s] DEBUG: Dropping privileges to uid 1000
[+33.87s] DEBUG: Writing /home/baldrick/.dmrc
[+33.97s] DEBUG: Restoring privileges
[+34.05s] DEBUG: Starting session ubuntu as user baldrick
[+34.05s] DEBUG: Session 3064 running command /usr/sbin/lightdm-session gnome-session --session=ubuntu
[+34.07s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session0
[+34.07s] DEBUG: Greeter closed communication channel"

On this occasion, it took about 8 attempts to finally bring up the login screen. After that, no probs.

I have 2 user accounts + the guest account.