I got disconnected and couldn't reconnect because someone was logged in on my account. Except, it was me logged in. The system should recognize my IP and the fact that I disconnected and allow me to log back in.
when you get disconnected and try to log in again immediately, the server sometimes still thinks your logged in, triggering this error message. If you try it again a moment later you should not get this message.
Problem solved, no a bug...
Which is weird, because when player is on blackzone, a character will stay for the next 1 minut on map after disconnecting, without ability to log back when (if) this message pop-up.