X gets killed immediately after successful graphical logon

["Uwe Dippel" <udippel@gmail.com>]

I can't get into any of my sessions, whatever I do.

The machine starts up and shows kdm, as it has been doing for some years.
Only, whenever I logon, as any user, or any session (kde, gnome,
xfce), it starts, then some scrambling of the screen, and back I am at
login applet.
The only one that works is Failsafe.
Therefore, it can't be a problem of a wrong password; it rather looks
like any X session started after the successful logon must have some
setting that kills X.
Unfortunately, the Xorg.log does not show anything extraordinary.

I wonder how to debug this further ...?

Uwe

Please, Cc: to me in case of answers; due to the traffic on this list