In another environment at our client site we get Error 500 -Unable to access the web page. But without the proxy we can access the page. Here are some test case scenarios we tested.
Following tests works fine
http://H02/rest/services/MyGPService/GPServer http://H02/iisstart.htm http://H02/rest/services/MyMapServices/MyMapService http://H01/MyApp/proxy.ashx?http:www.google.com
Following tests don't
http://H01/MyApp/proxy.ashx?http://H2/rest/services/MyGPService/GPServer http://H01/MyApp/proxy.ashx?http://H2/iisstart.htm http://H01/MyApp/proxy.ashx?http://H2/rest/services/MyMapServices/MyMapService
The "H02" server where AGS is installed is managed by their IT department and we don't have much access to it.
In the tests above, I cannot access any URL from "H02" server using proxy, even the root iisstart.htm. They all give Error 500, the page cannot be accessed.
It looks to me like permission or settings on "H02" server that is causing this issue. All the above tests works fine on our internal server.