Thanks, Rich and Arslan for your replies.
I have created an user 'kdm' with useradd.
Now I get a kdm login window and was able to login as user tc in kde.
The described error message is gone, but now another comes up
Mar 13 00:51:32 (none) daemon.err kdm_greet[12594]: Data directory "/var/lib/kdm" not accessible: No such file or directory
Mar 13 00:51:33 (none) daemon.err kdm_greet[12594]: Cannot load /usr/local/share/apps/kdm/faces/.default.face: No such file or directory
... but so far I had had no impact ...