Ok, thanks for your answers. Somehow this Ubuntu service came with an update of Ubuntu someday. I do not have a username nor password. My plan is to find that credentials, check whatever information is in this database and if it’s empty I’ll use it as my home automation base.
Ever since I used my method as described above no weird thing happened with Ubuntu. So I assume the Mosquitto database will be (almost) empty.
Just out of curiosity: What makes this Mosquitto service better than the HA MQTT broker?