An (old) colleague of mine brought this up with me so I thought I might write a post about it.

When trying to open login.aspx on a Forms Authentication site another error with no HTML error code appeared: "File Not Found" was the only message in the browser.

I found the solution on this blog post. The ASP.NET configuration tool within IIS Manager was adding xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0" in the <configuration> section of the web.config file. This was causing the "File Not Found" error and removing the xmlns property fixes it.

Hope this helps someone.