Comment # 5 on bug 1176229 from Dirk S.

Looks like kwin failed to start/crashed.
/usr/bin/kwin_x11 is running
Did you reboot after the last update?
yes
Does it work on a fresh user account?
yes
What happens if you run kwin_x11 in konsole inside the running user session?
I.e. what output do you get?
kwin: unable to claim manager selection, another wm running? (try using
--replace)
What displaymanager are you using?
I have no changed anything. I don't know.

I've rename the home directory and created a new one. Now it is working again.
Any idea what files should I compare to find out what happened?
Thanks!