Comment 6 for bug 1264887

Indeed I don't have kubuntu-desktop installed because I removed one (or more) of it's dependencies a long time ago. I think this is not uncommon, though. At the end of the upgrade I didn't let upgrade-manager clean up unused packages, therefore the reason for the removal of kdm seems to be something else.

For this upgrade test I made a copy of my main system to an extra partition and therefore I can try the upgrade again to verify if a possible fix works.