Have you ever edited your private boards.xml manually? It sounds like the file has been corrupted by an earlier change. If so, you wouldn't see any private designs when you start the program. If that's the case, just compare your private boards.xml with the one that came with the product (Program Files (x86)/gnubg) to see what the XML compiler doesn't like.
But what exactly did you do when you moved a copy of the file?