HTTP Error Behind ISA Firewall

I am trying to access a website and I keep getting this error message:

HTTP 502 Proxy Error – The Uniform Resource Locator (URL) is invalid. The request was not entered correctly. Enter the correct URL and try again. (12005)
Internet Security and Acceleration Server

It has something to do with my ISA Server Config. But I have the HTTP & HTTPS Ports open and accepting In and Out requests from the website I am trying to access. I can get to any website or secure site on the web, but this is the first time I have gotten this message from this particular website. I can get access through DSL at home but cannot behind an ISA Firewall. Can someone please explain the message and help resolve this issue in ISA.