Comment 3 for bug 67144

the following workaround fixed it for me: (warning to non-developers: do not use this workaround unless you know exactly what you are doing)

remove all 3rd party repositories from sources.list
sudo apt-get remove libgl1-mesa
sudo apt-get install ubuntu-desktop nvidia-glx

what that basically does is, revert lots of packages to their original ubuntu versions (and remove all open gl applications). after the apt-get remove the system doesn't even have gdm installed and the proprietary driver (nvidia-glx in this case) has to be reinstalled manually.

this bug might be a duplicate of
https://launchpad.net/distros/ubuntu/+source/update-manager/+bug/67103