Comment 3 for bug 1337960

Workaround:
sudo apt-get install --reinstall kubuntu-desktop

Still, the bug IS valid, since I am not the only one (see forum), and this isn't the first time (for me).
I have never asked for the kubuntu-desktop to be removed (then the everything in my previous attachment ought to have gone). What the command above did, was an installation of 4 packages altogether (sorry, I don't remember all 4, one was kde-artwork) of around 17 MB.

There is supposedly a bug that either removes a few important packages, or simply removes KDE (plasma) from the list of available desktop environments (DEs).
The hackish solution would be a wrapper that checks at the end of an 'upgrade', if the available and installed DEs are actually available to the display manager (I have no clue which one is being used here; earlier it was kdm, but it has been replaced by some earlier 6-monthly system upgrade).