[fysiks]

Is it always the same subset of players? If yes, then it's probably on them. If not, then it'll probably be really hard to diagnose.

You can attempt diagnose a potential plugin issue by disabling (in plugins.ini) all plugins that do anything with the player's names. Since it doesn't happen often (I think) you'll probably need to run the server like this for quite some time to be sure it's fixed or isn't.