There's an issue going around that would appear to be more common than I'd have thought. It allows players to log into any account while only knowing the account name.
It's exploitable on any server, provided they're vulnerable. Please see my - hopefully detailed enough - post to identify whether you're vulnerable and ensure it gets fixed. One guy abusing it's apparently blackmailing owners with it.
[Any version] Fix unauthorised access exploit to any account - Releases - Snoxd
@Players: Another good reason to avoid using the same account name as your character's name.