Seems like Grincher can corrupt save data. So, do not open any mail from Grincher, leave any room you see Grincher in, and do not join any room with a 1,000 player limit called "Grincher's Happy Room". This is not a joke.
Edit: Kicking Grincher from your game does not work, they are not affected by the kick action.
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They can also join your private lobbies without an invitation
Thanks, forgot to mention that!