Due to some changes in .Net 4.0, your existing tests will probably break. What's worse, they're breaking with the uninformative "400 Server unavailable" message. A quick fix would be to use absolute paths in your tests, e.g., instead of "Default.aspx" you use "/Default.aspx".

This exception usually happens when the Asp.Net engine cannot create an instance of an HttpContext. However, the inner exception is not provided (who needs it anyway?), so you're left in the dark. In this case, the actual exception happens inside the HttpRequest.ClientFilePath property getter. It now allows only absolute paths for some reason.

The fix is going to be released at some point next week.