openHAB 3.1.0 - mosquitto mqtt bridge not working anymore

Dear all,
i just updated from openHAB 2.5 to openHAB 3.1. and the connection of my mqtt bridge is not working anymore :frowning:
In openHab 2.5 it was working fine.
I am using a raspberry pi 3b+ and openhabian.
Error message in the gui is (translated)

The systemconnection with the name openhab_mqtt_rp3 does not exist anymore

However when i run the following in the rp3 terminal, everything looks fine:

[20:35:31] openhabian@openhab:~$ sudo service mosquitto status
● mosquitto.service - Mosquitto MQTT v3.1/v3.1.1 Broker
   Loaded: loaded (/lib/systemd/system/mosquitto.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2021-07-26 20:34:05 CEST; 3min 6s ago
     Docs: man:mosquitto.conf(5)
 Main PID: 660 (mosquitto)
    Tasks: 1 (limit: 2181)
   CGroup: /system.slice/mosquitto.service
           └─660 /usr/sbin/mosquitto -c /etc/mosquitto/mosquitto.conf

Jul 26 20:34:04 openhab systemd[1]: Starting Mosquitto MQTT v3.1/v3.1.1 Broker...
Jul 26 20:34:05 openhab systemd[1]: Started Mosquitto MQTT v3.1/v3.1.1 Broker.

I also tested the mqtt service, it works:

[20:44:39] openhabian@openhab:~$ mosquitto_sub -v -t 'test/topic'
test/topic helloWorld

However when i connect with my normal computer using the software MQTT Explorer i can connect but i get disconnected every 1-2 seconds (and then auto connected again).

Can someone help? Thank you!

Make sure that every single client that connects to Mosquitto has a different client ID. Continual disconnections are often a symptom of two clients connecting with the same ID.

I just created a new and it works now.
Seems like the first broker was setup as a systembroker. The new one is just a broker. Dont know what the difference is and why it worked before…

You don’t want a system broker with mosquitto at all, that was to do with managing the old OH2 ‘embedded’ broker. It didn’t “work” before, you did have some other broker Thing as well. But in importing both into OH3, something picked the wrong one to work with,