IIS6 Forms Auth and reverse proxies

Question

I've got a small problem. I have an apache reverse proxy serving to a backend IIS6 server. All sites work fine bar one. If I by-pass the reverse proxy the site works fine. If I go through the reverse proxy ( this is all in the same network and the site works fine directly from the proxy server ) I get back an http status : 400 Bad Request.

I'm guessing this is something to do with some form of cookie validation/checking or something.

Anyone have any ideas as to what settings I should change. I've tried enableCrossAppRedirects="true" and protection="None" but neither seems to have helped me.