Does someone know where I could start with this?
[2019-01-14 21:11:32] Connecting to MQTT broker …
[2019-01-14 21:11:32] Connection error with result code 5 - Connection Refused: not authorised.
You don;t give a lot of info, but it appears that your MQTT broker is only accepting authorized connections and openHAB is not delivering the correct credentials.
You may need to check the configuration of your MQTT broker and the Bridge definition in openHAB to make the required authentication match.
But I am no expert here and can likely not help you any further; I am running everything local and hence did not secure my broker…