This appears to only happen to certain people at random times, what will happen is a player will connect and authenticate but get's disconnected immediately and sometimes it will take the user to the character screen and display a disconnected from server message even though he is still at the character screen (should be kicked back to login screen)... Clicking the change realm button then selecting a realm will kick the user back to the login screen with disconnected from server.
I also found that when a player is attempting to connect (that unfortunately gets this issue) the server will print:
mangos>WorldSocket::HandleAuthSession: client 5875, unk2 0, account <snipped>, clientseed 3885476293
WorldSocket::HandleAuthSession: (s,v) check s: B6424EBD0663E4DECEE2ACFCBA669C9AA8064A214CC9787CE1F4456B3D0FB1EB v: 376AFBED6BAC240EEC3CA806C1ABC1962556323FF8AED467F44477CA1D7F0794
WorldSocket::HandleAuthSession: Client '<snipped>' authenticated successfully from <snipped>.
Server side warden for client 5 (build 5875) initializing...
C->S Key: BAEE44990BFE015BAD9173E2D6C61DEC
S->C Key: 7E23B0E5A0AAB4DCB00770516444D4C3
Module Key: AE25BC51063B77BD363C3EFE0FC173F9
Module ID: 79C0768D657977D697E10BAD956CCED1
Server Population (0.004000).
<none> (latency: 0, IP: <snipped>) exceeded Warden module response delay on state STATE_REQUESTED_MODULE for more than 10m - disconnecting client
This doesn't print to the logs, only seems to be visible in the server console itself...
The only fix I could find that works for a limited time before it happens to someone else is by restarting the realm.