In a coexistence scenario, in which the Coexistence Manager for GroupWise (CMG) Mail Connector is used, GroupWise users may not be able to accept or decline a meeting invitation sent from an Exchange user to a GroupWise user.
This is due to a bug with the Novell GroupWise 8.0 SP2 GWIA.
Novell has logged this as a defect 704520 (D101 error when Accepting iCal Appointments).
Install HP3 for GroupWise 8.0 SP2.
More information can be found here: http://download.novell.com/Download?buildid=gBjwGIdt77s~