I have this web application (ASP.Net MVC 3) that on certain conditions returns a custom JSON string with a HTTP status code for an error (403, 415, 500). It is deployed on a Win 2008 R2 server with IIS 7.5

Initially I was gettting the standard HTML pages for the error instead of the JSON data. I removed the error pages for these errors in the app settings. But now my queries which should return some JSON data return a single error line. When the server gives me 403, I have the message "You do not have permission to view this directory or page." (simple line, no HTML around it).

What can I do to deactivate this and finally get what the app is returning and not what the server wants to return?