When you get this error using TSWA, the solution is to recycle the Apppool of TSWA.
Go to your TFS Application tier and start IIS to recycle the pool.

But here at work, this error pops up, a few times each month.  So for a final solution read the TSWA Scalability Limits White paper: http://www.microsoft.com/downloads/details.aspx?FamilyID=ee378d7b-45c3-4690-bd06-a060cfa43b06&displaylang=en

All requirements described in this document, where OK for us.  But still we had the problem.  So we took the third solution in this document and every night we run a job that recycles the App_pool.  Now out problem is solved.