I am seeing this error in the system log as soon I configure Network Server Gateway to MQTT and subscribe via MQTT Client. At the same time, this error is triggering the MQTT client loosing connections as well for some strange reason.
In the LoRaServer (as the onboard version is called, the offboard version now being called Chirpstack) architecture, MQTT has two roles: in between the “gateway bridge” and the network server, and in between the network server and whatever is consuming the validated and decrypted data.
Your error message shoes that the gateway bridge is having trouble connecting with the network server, suggesting that you changed the configuration of MQTT for the gateway bridge or the gateway side of the network server, something you would only want to do if you were using an external network server.
If you are using the internal network server and merely want it to send the resulting data somewhere else, then you need to revert the configuration of the gateway bridge (possibly factory reset the gateway) and change only the application MQTT settings of the network server.