After Upgrade to OH3 MQTT / Mosquitto not working

Hello there.
After upgrading from V2.5 to 3 my mqtt broker isn’t working anymore.

Error:$AnnotatedConnectException: Connection refused: localhost/

My guess is, that there is a problem connecting to mosquitto. But the service is running. (Checked service status)

Probably the credentials are wrong. Unfortunatly i forgot the username/password I set in those days, so I tried so set it new. But how? Internet tells me to use mosquitto_passwd, but this does not exist in mosquitto 3.1 which OH installed during upgrade.

Any suggesstions? Uninstall and reinstall? Whats the best way to do this?

Has Mosquitto also updated itself? Maybe check this: Updating Mosquitto MQTT from 1.x to v2.0+

If this isn’t the issue you’re going to have to provide some more info on how you installed/upgraded openHAB, and how you installed Mosquitto originally.

I once installed mosquitto using openhabian-config.
For the upgrage to OH 3 I used the same.

Maybe, if no one has an idea, someone can tell me how to uninstall and reinstall mosquito?

Are there any log entries for that service in /var/log/mosquitto/mosquitto.log that may give a hint what is wrong ?