No (graphical) login possible.
 

I updated 14.04. Thereafter, when I try to log in the screen turns briefly black and then comes back to the login screen. No message about invalid passwords and such. /var/log/messages holds no "failure".

Where do I even look for a hint about the problem -- or even better a solution ;)?

Try the suggestions in this topic: http://www.linuxquestions.org/questi...ts-4175552161/

Hmm, thanks but I doubt this will help (can't try it right now, not at home).

Reason: the problem affects two users. A third can log in. I issued an "apt-get update" and "apt-get upgrade" frome there which ran without errors. The problem for the otheres is still there, though. But I'll try it as soon as possible.

so check ~/.xsession-errors and /var/log/Xorg.0.log

You were right, there were lots of permission denied errors in xsessions-errors. After some more discussions in German forums I got this link:

http://wiki.ubuntuusers.de/Homeverze...te-korrigieren

I issued all 4 commands, things are as they should be again :). One or several blasted pieces of software changed a few permissions in my /home directory tree. Problem solved, thanks to all participants here.

glad to help you
(if you really want to say thanks just click on yes)