I received the following error
Task Scheduler service has encountered RPC initialization error in "RpcServerUseProtseq:ncacn_ip_tcp". Additional Data: Error Value: 1721.
This prevented Task Scheduler, backup/restore points, defrag and other critical functions from working. It was the Avira Antivirus Premium 2012.
As soon as I uninstalled Avira, the TaskScheduler started working.