14w17a Account Conversion Failure

Updating from snapshot 14w08a to 14w10c, or 14w11b or 14w17a causes the following log attached to this bug report.

(Note: Multiple players are listed under IGN in the whitelist, op-list, ban-players list. )
I can conclude that the console wishes me to delete all player list text files because the new uuid system isn't able to reference the IGN's in those files properly.

If the player data folder is wiped some players have their stuff while others do not. Their location and inventories are reset.