I was trying to test multiple connections to my local IIS server when I got the http status/error code above. This is because there is a limit to the number of concurrent connections with Keep-Alive Enabled setting ON.

http://www.experts-exchange.com/Web/Web_Servers/IIS/Q_20286298.html

So I unchecked “HTTP Keep-Alive enabled” and the issue was resolved.

But when I run/debug from Visual Studio (for an application configured to run on IIS) i got an “Unable to start debugging on the web server. An authentication error ocurred while communicating with the web server…”. IMHO, the error messages doesn’t quite help in debugging but it turns out that Visual Studio just needs the the keep-alive setting ON to work for applications configured to run on IIS. Turned it back on and worked fine.