this kind of problems are usually caused by some IIS configuration issues, like application pool settings, application pool anonymous settings, in my case, I set application pool anonymous settings user to application pool identity, and application pool process model to LocalSystem and solved the problem, this is a right case, there was nothing wrong with my web.config file or anything about the bits, so, just change the settings.
As to IIS settings, trouble shooting always locates at application pool and access authority related, including the .NET framework version. Check these settings and restart your site. Just dash-