typically this indicates the wrong URL is being called, many times that's caused by the route being mapped to a different endpoint. Can you please post the service route mapper code and the url you are trying to call e.g.

mapRouteManager.MapRoute("MyServices", "{controller}.ashx/{action}",new[] { "SDNServices" });

in this case, the URL would be something like the following, where the first part matches the MyServices part, my controller is called WelcomeController and has a public HelloWorld method. In additinm the WelcomeController is within the SDNServices namespace

http://localhost/test1/DesktopModules...