UPDATE - Please look at my answer for a link and explanation of the solution to this problem
Before we start, I know this is a very common quest
I got the same error message after getting the MVC3 from NuGet.
I don't know if you got your project setup the same way as I do but my web.config is auto generated depending on environment. The MVC from NuGet is adding rows ( to the web.config and those rows were removed since my merging config-files wasn't setup correctly.
Regards,
Kim