Laszlo has checked the offending MP3 for me (thank you for that) and has discovered that the problem is that the MP3 is encoded to 22KHz. I have thousands of MP3s and I'm sure there are maybe a few at least that could potentially be like this.
File type is not compatible, that's fine. But surely locking the whole system up is not fine at all - Pioneer can you please respond to advise if there is anything that you planning to do to stop an incompatible file from rendering the unit useless until it is either restarted or unlinked from rekordbox?