I was experiencing the same error, although intermittently rather than all the time. After a reboot it may go away, but then come back hours/days later. However, its now got worse - the Forefront TMG Managed Control Service fails to apply the email configuration and then stops. I can't get the service to start at all.
TMG with Exchange 2010 seems to be very problematic. I had TMG running for weeks on another server with out Exchange 2010 and it worked flawlessly.