HI guys,
Sorry for the late reply.
Well guess what, the problem was caused by the time difference between the two (client and server). There was (for a reason that I ignore) a difference of 12 hours back time on the server... As soon as the correct time has been set on the server, the client could connect in the workgroup env. normaly.
I read after that a difference exceeding 5 minutes will stop the kerberos authentication, rendering thereafter any login attempt impossible.