This was a very frustrating error, which costed me the better part of my day.
When I wanted to restart KDM, I got the login screen where I entered my userid and password. X would show the hourglass for a few seconds and then the login screen re-appeared.
The ~/.xsession-errors file contained only these lines:
Xsession: X session started for jlinkels at Fri Sep 1 08:44:38 AST 2006
open: permission denied
and the /var/log/syslog told me this:
Sep 1 08:53:44 localhost kdm_greet: Can't open default user face
This is an error which is reported quite often, but hardly a working solution is found.
In my case the reason was that the /tmp directory permissions on my machine were drw-r--r--
while it should have been drwxrwxrwt
The command for changing it is:
chmod ug+rwx,o+rwt /tmp
After changing the permissions KDM worked fine again.
What confused me is that /etc/X11/Xsession script does contain a line which does a check for writability of /tmp, however I did not find the error message that Xsession oughts to generate. So I assumed that it was not the writability of /tmp.