"Unable to Start Debugging on the Web Server" error when hosting your web application on your local IIS

It works fine when you host your project using the Visual Studio Development Server, but when you choose IIS, you get this error.  Try the following from a command prompt:

%SystemRoot%\Microsoft.NET\Framework64\{version number goes here}\aspnet_regiis -i

I think this happens when IIS doesn't recognize the version of the .net framework that you are running.  So this might happen when you install a Service Pack for instance.  Anyway, this line of code makes the version known to IIS.  Keep in mind that if you are running version 3.5 that you find this aspnet_regiis executable in the folder for version 2.0.  3.5 shares a common CLR with 2.0.