Thank you for your answers.
@dirkfanick:
gdm or gnome is not an option just because of this KDE problem.
There are no unstable packages or unsupported compiler flags - I am using only the stable versions, because the machine is for "productional use" (=my wife also uses it)
@Andy
/var/log/kdm.log shows some new messages after KDE upgrade (befor KDE upgrade only the xkbcomp messages were present), hoewver these messages always appear, regardless of KDE starting up completely or not. The messages in kdm.log are:
---BEGIN---
klauncher(5191) kdemain: No DBUS session-bus found. Check if you have started the DBUS server.
kdeinit4: Communication error with launcher. Exiting!
kdmgreet(5185)/kdecore (K*TimeZone*): KSystemTimeZones: ktimezoned initialize() D-Bus call failed: "Not connected to D-Bus server"
The XKEYBOARD keymap compiler (xkbcomp) reports:
> Warning: Type "ONE_LEVEL" has 1 levels, but <RALT> has 2 symbols
> Ignoring extra symbols
Errors from xkbcomp are not fatal to the X server
--- END ---
Xorg.0.log looks like before the KDE update - no anomalies or differences there.
"top" tells me that plasma-desktop consumes 100% CPU when the failure occurs. Google finds a lot of hits for 100% CPU consumption of plasma-desktop but none of them is related only to startup (when KDE runs, I never got an issue of 100% CPU of plasma desktop - it is always just at start)
A completely new profile is not fun because I have to set up all things of KDE again. I now copied only ~/.kde4/share/config/kwinrc from a new and clean profile, however it does not show differences that should be relevant (mainly: compositing is disabled). I will see whether this helps the next days...
tom