From time to time, the UPS fails with the following symptoms:
- The event viewer (application log) displays the events 6801, 6803 and 6110, in this order.
- The Forefront Identity Manager (found here: C:\Program Files\Microsoft Office Servers\14.0\Synchronization Service\UIShell\miisclient.exe) shows failed synchronizations with the status: stopped-extension-dll-exception.
- And of course, updates between SharePoint and Active Directory are not sent anymore.
The best clue is found in the details of the eventID 6801.