I use TMG as a reverse proxy to redirect SSL traffic based on URL using a third party wildcard certificate. I've configured this over other devices (namely unsecured HTTP pages for internal devices, NAS, UPS, etc) and other Windows products. When configured with WHS (installed the WHS self-signed certificate on the TMG) and can access the WHS remote access site and it renders the home page, but not the Media Library portal and when I access the Share Folders it returns an error.
There is nothing in the WHS event log and TMG is reporting Alllowed. I've also disabled the HTTP filter rules that can trap extended traffic.