I hope someone can help me.

I've got a strange issue here at work:
One user can't edit the sharing permissions for his own primary/
standard Outlook 2003 Calendar. The sharing menu can be openend,
actual entries get displayed but as soon as any changes are made and
want to be saved a message (translated from German) "The changed
permissions coult not be saved" (German: Die geänderten Berechtigungen
können nicht gespeichert werden.) appears.

I've checked the mailbox rights on the exchange side, everything fine.
I gave myself fullaccess to the mailbox and openend his exchange
mailbox on my own outlook, same problem.
Abou two hours of research on the net gave no additional hint on how
to solve this issue, and I want to avoid, if possible, deleting/
recreating the mailbox.

I'm not sure if I can use a switch now, to reset the permissions to
the defaults (if that is even possible) or if I have to edit something
on the exchange side.

I would appreciate some help

Environment Specs:
- Windows XP SP2 English with German, French, Italian MUI
- Outlook 2003 SP2 (Build 11.6568.6568) (Office 2003 Standard, SP2,
english with same MUI as OS)

- Around 80 Users, only user with that problem (AFAIK)

- Windows Server 2003 SP2 Enterprise
- Failover Clustered Exchange Enterprise 2003 (6.5 Build 7638.2: SP2)

Greetings Sebastian