The wierd thing for me was that, as when I installed LR I had already deleted "My Pictures" folder, I started by pointing LR's database and backup to another folder I created.
I have LR's files on this new folder, but still had to "rebuild" My Pictures to have LR exporting functionable, even though it just stands there, empty, with no function at all.
As I see your problem is not probably what mine was, but there might be some folder issue, some pointing issue, who knows?
Sorry it didn't help!