To reset the Collection go to Menu > Options > Sync Settings. Unselect Calendar. Hit Menu > Save. This will now remove the calendar data from your device (but NOT the server). If you have a large number of items this may take a while.
Once items have been deleted, reboot the device (remove battery). After the device has come back online go back into AstraSync, Menu > Options > Sync Settings and reselect the option to Sync Calendars. Hit Menu > Save. If you get any warning about existing items being on the device, the suggestion action is to 'Delete Device Items'. AstraSync will now sync the items from the server back to the device.
Further Information:
This situation occurs when AstraSync detects that more than 50% of events in your calendar have been deleted. Generally when this situation occurs it is due to some error. To prevent data loss, AstraSync goes into this error state and requires you to reset the collection.