Symptoms: Upon logging in, the effected player 'might' be asked to re-register their account. Whether they do so or not, it appears that their data is missing from the database.
This is a known issue, I spent months looking for an easy fix. Beyond scrapping the current MySQL stuff and paying close attention to variable resets, I have no suggestions. This has effected quite a few players in the last few days.
Good luck Stevo!