API restriction: The assembly <WindowsBase.DLL> has already loaded from a different location.
The difference is mine was for the WindowsBase.dll. I tried removing the reference, restarting VS.NET, redoing the reference, restarting my machine, nothing wanted to work. I was getting totally p!$$#d off!
So I created a new WPF app, attached all the files from my old app and ….. It worked. Hope this helps someone else.