Hi All
The Problem is now finally solved with Software Update 1 for TMG 2010 SP1 http://www.microsoft.com/downloads/en/details.aspx?FamilyID=695d0709-0d8b-45ee-afdb-727c4428ca4d
I have also Updated the Exchange 2010 SP1 (Edge Role) on the TMG http://www.microsoft.com/downloads/details.aspx?FamilyID=50b32685-4356-49cc-8b37-d9c9d4ea3f5b&displaylang=de
No more errors since then :o)
Regards
Andres
-- MCSE 2003 MCSA 2003 Messaging MCITP: Enterpise Administrator MCTS: Windows Server 2008 MCTS: Exchange Server 2007 Configuration MCTS: Microsoft SQL Server 2005 VCP - VMWare Certified Professional