@Lukas > Chances are the 8302 is because of a communication issue between rekordbox and the player. Please ensure that there are no other networking items enabled (file sharing, etc.) and verify that your wireless is disabled. If you can check for certain that your firewall is off and no 3rd party firewalls are running as well.
Try with one player connected directly to the ethernet connection.
Are you running a MBP retina with an ethernet adapter?