I had the same problem, i believe it's caused by a large number of databases,
How many databases do you have?
my SQL 2008 server has 1294 Databases, i believe that the server is running out of available threads specified by the 'Max Worker Threads' advanced property,
i'm going to try increasing it to 1300 and see if that helps
http://social.technet.microsoft.com/Forums/en-US/dpmhypervbackup/thread/43c7086d-c718-45ac-8ce6-79825fd56ad7/