If you’re getting this error when trying to debug an
ASP.NET Web application on
IIS7 or greater, check the
system.webServer element in your
Web.config. If you have the
httpErrors element configured, you won’t be able to debug. For your local dev environment, remove or comment out the
httpErrors element, and you should be good to go.
From my ASP.NET 1.1 days, I have a hazy memory of Visual Studio’s debugging startup process requesting a bogus URL on the target Web site, and then doing something with the 404 response prior to launching the debugging session. Obviously, if it’s expecting a 404 or the page to contain certain text, redirecting to an error page will prevent you from launching the debugger successfully.
If you happen to know specifically why this happens, please leave a comment (or contact me if comments are closed), and I will gladly update this entry.