Problem:

Solution:

Pressing iSync's "Reset Sync History" button not only resets the settings on the devices that are sync'd via Sync Services, but also tells OS X's Sync Services to "forget everything it knows about registered applications without unregistering them" (technically it puts them into "refresh mode") which causes Groupcal to think it's never been sync'd.

The Knowledgebase article I've tried everything else but still can't complete a sync operation ... what now? advises on situations where iSync's "Reset Sync History" button can be useful for solving problems that affect Groupcal's operation.