I am having problems setting up SSL VPN web proxy applications with our 2960.

I have enabled SSL access, created a test user, and created an SSL Web Proxy profile for Outlook Web Access. In this profile, I've entered the URL for OWA (https://webmail.company.com/owa, which resolves correctly both internally and externally. The Host IP Address is set to the internal IP address of the Exchange box.

When I login to the web proxy with my test user, the OWA link is presented to me as expected. However, if I try to open it, I get an HTTP500 error.

What could be causing this?