Yesterday SharePoint greeted me with this error message when I tried to create a new site inside a site collection:
File not found.
I examined the ULS logs of the SharePoint server and I noticed that it tried to open the new web by name and it failed. The reason was a faulty Alternate Access Mappings entry, as shown here:
Exception type: System.IO.FileNotFoundException
Message: The Web application at http://whatever.domain.com/newsite could not be
found. Verify that you have typed the URL correctly. If the URL should be serving
existing content, the system administrator may need to add a new request URL
mapping to the intended application.