Have a classic ASP site running on .Net 2.0 framework. IIS Rewrite rules are defined in the web.config, to allow all friendly URLs. I just switched to .Net 4.0 in our dev environment, and all requests now result in GenericErrorPage.htm?aspxerrorpath=/404.asp. The only thing that was changed is the framework for the application pool, from 2.0 to 4.0. Switching back to 2.0 from 4.0, the page load fine again.

Any idea why this happens?