Only two players can not receive messages from others

[Volc94]

Hello,

We're having an issue where only two players (myself, and another) are unable to receive messages. The message sender is told "Player: volc94 is not online". I certainly am online. I'm not op'd, not vanished, and I've had every single permission node removed to help try and isolate what's causing this, but still, the "Player: volc94 is not online" is being sent.

We are using VelocityVanish. The dev doesn't believe this is coming from their end.

Any ideas?