Recently I have encountered strange TMG behavior. I have permitted all RPC connections to Domain Controllers with system rule and disabled RPC Filter plus switched off “Enable strict RPC compliance” option. But my WMI requests and any other RPC traffic from TMG servers to internal resources was still blocked.
The error was “RPC Server Unavailable”. Some programs gave me error code 1722. The problem was in understanding of traffic direction for access rules containing “Local host”. To tell you the truth I still do not understand why it works such way. Even after I’ve read http://tmgblog.richardhicks.com/2011/12/05/forefront-tmg-2010-protocol-direction-explained/. So the short answer to fix this problem – you need to create new rule allowing RPC traffic with SOURCE=Internal and DESTINATION=Local host. Also after creation you need to disable RPC filtering for created rule.