[Vince]

Your server is sending too much data to the player at once. The client must send an acknowledgement for each update back to the server. The limit was introduced because there are exploits that send fake acks. The general advice given here is to raise the ackslimit but that's merely a workaround and not an actual fix.

One notable cause is toggling spectating off within OnPlayerSpawn. Doing so will call OnPlayerSpawn again and as such create an infinite loop.