Guest session not working

I could not reproduce this on an updated 32-bits saucy.

That's strange, it is completely reproducible with mine (64-bit). What log file would have the details about the guest session's startup?

The guest account creation script has failed (it's a shell script so no useful messages as to why), could you run

$ sudo /usr/sbin/guest-account add

And see if that works?

Looks like it was a leftover lock file. Your command returned:

Adding system user `guest-Os5HCE' (UID 197) ...
Adding new group `guest-Os5HCE' (GID 248) ...
Adding new user `guest-Os5HCE' (UID 197) with group `guest-Os5HCE' ...
useradd: existing lock file /etc/subgid.lock without a PID
useradd: cannot lock /etc/subgid; try again later.
adduser: `/usr/sbin/useradd -d / -g guest-Os5HCE -s /bin/bash -u 197 guest-Os5HCE' returned error code 18. Exiting.
umount: /tmp/guest-Os5HCE: not mounted

I have removed /etc/subgid.lock and now all works as expected. How can this lock file be left over? Perhaps leftover locks should be deleted at boot time.

Thanks, Robert, you've solved my problem!

Reassiging to the passwd package which provides "useradd". There doesn't seem to be any command line option to force it to creat e the user account.

This bug was fixed in the package shadow - 1:4.2-3.1ubuntu1

---------------
shadow (1:4.2-3.1ubuntu1) xenial; urgency=low

  * Merge from Debian unstable.
    - Includes pam_loginuid in login PAM config. LP: #1067779.
    - Fixes typo in usermod -h output. LP: #1348873.
  * Remaining changes:
    - debian/passwd.upstart: Add an upstart job to clear locks on
      [shadow-]passwd/group.
    - debian/login.defs:
      + Update documentation of USERGROUPS_ENAB: with pam_umask, the UPG
        handling does not only apply to "former (pre-PAM) uses".
      + Update documentation of UMASK: Explain that USERGROUPS_ENAB
        will modify this default for UPGs.
    - debian/{source_shadow.py,rules}: Add apport hook
    - Pass noupdate to pam_motd call for /run/motd.dynamic, to avoid running
      /etc/update-motd.d/* scripts twice.
    - debian/patches/1010_extrausers.patch: Add support to passwd for
      libnss-extrausers
    - debian/patches/1011_extrausers_toggle.patch: extrausers support for
      useradd and groupadd
    - debian/patches/userns/subuids-nonlocal-users: Don't limit
      subuid/subgid support to local users.
  * Dropped changes, included in Debian:
    - Allow LXC devices (lxc/console, lxc/tty[1234]), used from precise on.
    - Add uidmap package based on upstream patches that introduce
      newuidmap/newgidmap as well as /etc/subuid and /etc/subgid. Additional
      updates on those to widen the default allocation to 65536 uids and gids
      and only assign ranges to non-system users.
    - debian/patches/1020_fix_user_busy_errors: Call sub_uid_close in all
      error cases.
  * Dropped changes, included upstream:
    - debian/patches/495_stdout-encrypted-password: chpasswd can report
      password hashes on stdout.
    - debian/patches/496_su_kill_process_group: Kill the child process group,
      rather than just the immediate child.
  * Fix pam_motd calls so that the second pam_motd is the noupdate one rather
    than the first, ensuring /run/motd.dynamic is always populated and shown
    on the first login after boot. LP: #1368864.
  * Don't call 'pam_exec uname', a change adopted in Debian without
    coordination with the Debian PAM maintainer
  * Use dh_installinit now for installing the upstart job, as we no longer
    generate a dependency on upstart-job.
  * Include /etc/sub[ug]id in the list of files to clear locks for on boot.
    LP: #1304505
  * Add a systemd unit to go with the upstart job, so that lock clearing works
    on newer Ubuntu releases.

shadow (1:4.2-3.1) unstable; urgency=medium

  * Non-maintainer upload.
  * Fix error handling in busy user detection. (Closes: #778287)

shadow (1:4.2-3) unstable; urgency=low

  * Enforce hardened builds to workaround cdbs sometimes not building
    with hardening flags as in 1:4.2-2+b1
    Thanks to Dr. Markus Waldeck for pointing the issue and Simon Ruderich
    For providing a working patch.

shadow (1:4.2-2) unstable; urgency=low

  * The "Soumaintrain" release
  * The "Rigotte de Condrieu" release was 4.2-1
  * Upload to unstable
  * Last upload integrates the use of dh_autoreconf which has the same
    effect then Eric Dorland's patch in 1:4.1.5.1-1.1 ...