I am getting the same error, I have tried to setup the bridge on the gateway itself, so when I try to connect to aws iot core i get this error on the gateway.
I do not see any other process running on the same port or the gateway.
Could you please help .
After updating my Ubuntu 18.4 to 20.4 i suffered the same problem and the caus was that the packet-multiplexer now started before the gateway after the gateway was unable to start. Disabliing the packet-multiplexor for which i was unable to find any dokumentation on everything now starts up fine.