As far as I know, such thing happens if the server is started with a machine_id= option differently to what it was before. That means, if you previously started your server with machine_id=something, and now you use machine_id=someotherthing or no machine_id at all, or vice versa, this will happen. This is an option either directly on the server start command line or an option in the *.ini file if you use an ini file at server start.