Bad Gateway using Outlook Web Access with Reverse Proxy

[KWIK]

Hello all, I have a little problem with a 'Bad Gateway' error on our reverse proxy.

I am running Apache on a Debian machine which is used to access an outlook web access client, we have a Cisco firewall in the path of the connection. I have checked this and the hit counters are incrementing, meaning that the connection is going through, I can also telnet to the box itself on port 443. My Apache configuration is correct as it was previously working. I was wondering if it could be configuration error on the client side. Unfortunately I do not have control over the back-end server and do not know the configuration of the OWA access for that box. I need to know if there is any common issues, or if there is anything that stands out to anybody. All help would be most great full. I could probably get some information from the owners of the exchange server, if needed. Many thanks.