If a page element is missing—either a user control hasn’t been deployed or something else that needs to be in place for the page to compile dynamically is awol—you might be the proud owner of a 404 even though the page itself does exist.
HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.
After setting the SafeMode element’s CallStack element true in web.config, turning off CustomErrors, and setting the debug=”true” attribute on the compilation element, you’re possibly looking at a beautiful ASP.NET error message with no stack trace and not a lot to go on.
Well here’s a trick: view source on the error page and and scroll down below the closing HTML tag. Behold the stack trace! It should tell you exactly where to look for the problem.