ASP.NET IIS 7.5 HTTP 500.21 error
This error occurs when ASP.NET is not installed correctly on the computer that you are trying to debug. This might mean that ASP.NET was never installed or that ASP.NET was installed first and IIS was installed later.
In order to register .NET with IIS. Just type from a command prompt window, run the following command:
where version represents the version number of the .NET Framework installed on your computer, such as v1.0.370. You can determine the framework version by looking in the \WINDOWS\Microsoft.NET\Framework directory.
Example (full command)
C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe -i C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe -i