IIS 500.19 error on ASP.NET 4.0 sub-application

I've created a "test" application below the main application on my website which is an exact duplicate of the main application - I use the /test app to test new changes before putting them in production.

After upgrading the app from .NET 3.5 to .NET 4.0, my /test application refuses to run, returning a 500.19 error from IIS. My main application (and another sub-app at the same level as /test) runs just fine.

I found a lot of references about the 500.19 error and they usually boil down to some line in the web.config that is invalid.

However, in my case the error doesn't point me to any specific line -- the error message is copied below, note that the "Config Source" section is empty.

Error Summary
HTTP Error 500.19 - Internal Server Error
The requested page cannot be accessed because the related configuration data for the page is invalid.


Detailed Error Information
Module IIS Web Core
Notification BeginRequest
Handler Not yet determined
Error Code 0x800700b7
Config Error 
Config File \\?\c:\blahblahblah\test\web.config
Requested URL http://localhost:80/blahblahblah/test
Physical Path C:\blahblahblah\test
Logon Method Not yet determined
Logon User Not yet determined
Config Source

   -1: 
    0: 

Any clues would be appreciated!

Glad you're sorted now.

For me (and these guys) this error was a due to the URL Rewrite Module not having been installed - look for a 'rewrite' section in the web.config.