You try to build your project in Microsoft Visual Studio 2013 and you get the following error.
You have even rebooted the computer and this problem returns.
There are some windows open with a resource (most likely a config file) related stuff open in Visual Studio 2013.
The reason why rebooting would not work is that Visual Studio keeps track of the session state next time you reopen the project that causes the problematic document open again.
Close all the documents and rebuild (yes, it's just as simple as that!).