title: Adding 401 custom error to system.webServer.httpErrors gives "Unable to start debugging on the web server" error
body: <p>I started with a blank MVC 3 web application. I created a new website in my local IIS for localhost:80 and configured my new MVC 3 project to use that web server instead of the VS development web server. At this point I can run and debug the project as normal. But if I add this to my web.config, I get an "Unable to start debugging on the web server" error. I can select Start without debugging and it works fine. What's the deal?</p>
  
  <pre><code>&lt;system.webServer&gt;
      &lt;validation validateIntegratedModeConfiguration="false" /&gt;
      &lt;modules runAllManagedModulesForAllRequests="true" /&gt;
      &lt;httpErrors errorMode="Custom" existingResponse="Replace"&gt;
          &lt;remove statusCode="401"/&gt;
          &lt;error statusCode="401" responseMode="ExecuteURL" path="/Error401" /&gt;
      &lt;/httpErrors&gt;
  &lt;/system.webServer&gt;
  </code></pre>