There's no telling what might go wrong with a kb or why ...
A few years ago I was experiencing a 'lock-up' with my Pa900 (hmmm, maybe it's something about the '900'number) ... I got in touch with KORG USA and they helped me track it down to when I was searching for files on the USB ... I believe I sent them a copy of the USB files, and there was a corrupted non-KORG file on the USB that was causing the problem ...

Hopefully, YAMAHA will be able to replicate the problem and find a solution ...