Dear Simon et al., thank you very much for your answers!
Do you have GLib 2.25.12-1 from experimental, which is known to be crashy (<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=591075>)?
Yes, you are exactly right. I forgot to mention in my first post that the package mixture on my systems spans from testing over unstable to experimental and indeed I had libglib from experimental installed for some reason I don't remember. As soon as I downgraded to the version in unstable again, I had a working xorg back. So this post is merely a duplicate of #591075.
information for developers; if reportbug is among the things that crash, it's possible to use any other email program, although the resulting bug report will need more input from you (see http://www.debian.org/Bugs/Reporting).
Yes, I know, but I had no idea which package to file the bug against and so I hoped to get some hints from posting to -devel. You just gave this hint, but since the corresponding bug report has already been filed this issue is closed for me.
BTW, reportbug seems to have died, because I had the frontend set to GTK2. Now it also works again.
Thanks again, - Fabian