Internal Server Error 500.21 In IIS

It means ASP.NET is not properly registered with IIS. In order to resolve the problem follow below steps:

1. Open command prompt in Administrator mode. Search "cmd" and right click on it to select Admin mode.
2. Type the line in the command prompt below any one

%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe -i

or

%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe -i

http://www.winsysadminblog.com/2011/08/fixing-http-error-500-21-internal-server-error-handler-webservicehandlerfactory-integrated-has-a-bad-module-managedpipelinehandler-in-its-module-list-on-iis/