ADFS though TMG - connection reset

Hi All,

trying to publish ADFS through TMG and am running into a weird problem. Everything works internally and even when VPN'ed in, but no connection through TMG is possible.

TMG logs show connection resets (Error 64 - Network Name no longer available). I did a NetMon trace on the ADFS server and can see that the traffic from the external client (or from TMG when doing a "Test Rule") hits the ADFS server, but the server immediately sends a connection reset back (Frame 73).

I can only guess that it's the application (i.e. ADFS), that's terminating the connection, but why?

Anyone seen this before?
Thx in advance for help and/or pointers!
M.

Note: the blacked out stuff is just "noise" (=other connections)