Client in caching mode reports 'has encountered a problem with your mailbox.... rebuild...

Hi All

I have a user here that keeps getting a message in his caching mailbox.
"GroupWise has encountered a problem with your mailbox. GroupWise can
rebuild your mailbox to avoid further problems. Rebuild now (this may
take several minutes)?"

No matter what he does, rebuild or not, it reappears after a week.
He has syncronizing via GWMobile to his Nokia phone enabled.

I've done both a content check and fix and a structural check and fix
on the POA for his account, no problems reported.

What to do?

--
Best regards
Fribse