OSX and Caldav -> Error 403 and 404

[peko]

Hi,

a new user has joined our institute. Trying to access our OX 6.20.7 with ICAL
an errror 404 is alway occurring.
The DAV-Log is only showing
dav_443.access.log:141.5.YY.XX - - [09/Jan/2014:08:04:25 +0100] "PROPFIND /principals/users/USER/ HTTP/1.1" 401 294
dav_443.access.log:141.5.YY.XX - - [09/Jan/2014:08:04:25 +0100] "PROPFIND /principals/users/USER/ HTTP/1.1" 207 1417
dav_443.access.log:141.5.YY.XX - - [09/Jan/2014:08:04:25 +0100] "OPTIONS /principals/users/USER/ HTTP/1.1" 200 -

For other users it's working. I also do not see anything special in the user settings (listuser --csv).

Any idea ?

Bye, Peer