unattended-upgrade-shutdown crashed with IOError in _open(): [Errno 2] No such file or directory: '/var/log/unattended-upgrades/unattended-upgrades-shutdown.log'

This is easily recreated on an EC2 image (likely elsewhere), the user just needs to do:
  sudo service unattended-upgrades start
  sudo service unattended-upgrades stop

To me it seems like the real problem is that the directory the file is supposed to be in isn't created if it is missing

sudo mkdir /var/log/unattended-upgrades

should fix the problem. Can anyone confirm that?

Probably i have this problem because my var/log is in ram, because i have an SSD.

As Michele Olivo mentions I too use tmpfs for var/log, so there might be something there. Still this should not happen.

i too use tmpfs as i have an ssd. seems like that's the problem

Yep, same for me. I have SSD and use tmpfs for var/log.

I temporarily palliated by adding the following line to my fstab, seems to have fixed it for now:
tmpfs /var/log/unattended-upgrades tmpfs defaults,noatime 0 0

I am using an SSD with /var/log mounted in tmpfs as well.

Still happens in beta1 as of today.

This happens just now again when first logging in for the day.

am using ssd with tmpfs, I am sure this occured several versions ago and was fixed, it seems to have reappeared.

This bug was fixed in the package unattended-upgrades - 0.79.3ubuntu3

---------------
unattended-upgrades (0.79.3ubuntu3) quantal; urgency=low

  * lp:~mvo/unattended-upgrades/lp1061498:
    - fix crash in dpkg_conffile_prompt() (LP: #1061498)
  * lp:~mvo/unattended-upgrades/create-logdir-in-shutdown-helper-lp1013363:
    - create logdir in the shutdown helper if needed (LP: #1013363)
 -- Michael Vogt <email address hidden> Fri, 05 Oct 2012 17:57:38 +0200