Why are blank pages being served with "200 OK" for asp.net errors in IIS 8.5 (Win 2012 R2)?
Had a similar issue on Windows 8.
In settings search for "Turn Windows features on or off".
Check that the following features are enabled
"/Internet Information Services/World Wide Web Services/Common HTTP Features/HTTP Errors". "/Internet Information Services/World Wide Web Services/Common HTTP Features/Static Content".
You need to ensure Server Side Debugging is not enabled in the ASP module.
Classic ASP server 500 errors are returned as 200's. An attempt is made at opening the Server Side Debug Application, that then can't be found and IIS subsequently returns a 200 response and a blank page.
Make sure that you are not calling Server.ClearError() in Application_Error of Global.asax.cs that ended up being my problem.