Riva stops syncing, and the CRM Monitor application reveals the same error in every user's activity monitor window:
An examination of the most recent crmex-log file in the \Riva\Logs folder reveals the following error:
2011-12-19 15:13:41,263 ERROR [CTP-yxd #1]  Minor error occurred during synchronization.
Omni.Scheduling.SchedulePostponeException: Unable to connect to remote Exchange server with given credentials. ---> System.UnauthorizedAccessException: Unable to connect to remote Exchange server with given credentials. ---> Microsoft.Exchange.WebServices.Data.ServiceRequestException: Request failed. The remote server returned an error: (401) Unauthorized. ---> System.Net.WebException: The remote server returned an error: (401) Unauthorized.
Cause of Connection Failure
Riva cannot connect to the target Exchange system configured in the Exchange connection object, because the credentials of the Riva connection account are invalid. The most probable cause is that the AD/Exchange password for the Riva connection account has been changed. If the password is changed in AD/Exchange without changing the password in the Riva connection object, Riva cannot connect to the target Exchange system.
Change the password in the corresponding Exchange connection to match the password for the Riva connection account in AD/Exchange: