Why am I getting the message "The specified request cannot be executed from current Application Pool"?

Quite not sure why I see this error. I navigate to my Login View like so http://test.staging.com/mywebsite/Login

My Login view was just redone using MVC but I have seen this same error message going to an aspx page as well...

If I use http I get the error message The specified request cannot be executed from current Application Pool. If I use https://test.staging.com/mywebsite/Login, I'm good. If I don't specify a protocol, test.staging.com/mywebsite/Login, I get the error as well

Is there an error happening under the covers and my custom error page can't be shown like discussed here?

What are some other causes of this error?

Solution

It could be because you're using different versions of ASP.NET for one or many apps in the pool.

Make sure all apps in the pool use the same version of ASP (e.g. ASP 2.0.50727)
If you just added a new app, try changing the app momentarily to a different version of ASP, then back to same version. I experienced an issue where the displayed version was correct, but under the hood, a different version was used!
Check your event log, under Application, to get more details about the error.