After running a zypper dup and rebooting, KDE didn’t start anymore. Black screen, mouse pointer, that’s it.
After some trying I was able to get XDM to start so I was able to login and kde was starting again, I was able to use it.
However, after some minutes the system froze. Turns out, that a script /usr/etc/X11/xdm/sys.xsession is called when logging in and this script does a recursive call on itself. After some minutes there are thousands of processes until the system finally freezes. If I kill all these instances my session is killed, too, returning me to the xdm login screen.
Anyone an idea what is going wrong or had a similar issue? Before rebooting everything was working like a charm.