Newly introduced in Transport 2.1 is the maxMessageSize parameter found on NetworkSettings, a customizable max packet size for restricting the size of packets other than 1400 bytes what was previously hardcoded into NetworkParameterConstants.MTU.
Netcode’s CommandSendSystem does not yet recognize MaxMessageSize so changing this value will cause the send system to break.
Please change CommandSendSystem (specifically line 181) to instead use the MaxMessageSize parameter on the driver’s NetworkSettings.
Just in case anyone stumbles upon an error caused by reducing the max message size and googles it:
Failed to send a snapshot to a client with BeginSend error: -4!