I could fix prevent kdm from crashing by installing "xfce4" and setting the alternatives
- x-session-manager to /usr/bin/startkde
- x-window-manager to /usr/bin/kwin
it do not make sense but it's working. I got to it, as my NFS-server wasn't up and I only could login to "failsafe"-mode. And here KDM did not crash on logout. After installing "xfce4" (xession-manager and window-manager were set to the xfc alternatives) logout worked also. And restoring the managers to the kde-alternatives did ot break the logout-function.