SOLVED!
[quote=“Dim, post:4, topic:31993”]
is your broker alive and kicking? use a tool like mqtt.fx to connect to it and try some publish/subscribe stuff to test it out.
if it’s working ok: check if you have installed the MQTT Action addon (remove it)
[/quote]connected mqtt.fx with Broker Address = openHAB2 ip (where mosquitto runs, too)
then on “broker status” i can subscribe to “Mosquitto” or “Hive MQ”, when i subscribe to Mosquitto i can see some messages regarding my sonoff device like:
2017-07-21 20:44:28,519 INFO --- MqttFX ClientModel : messageArrived() with topic: tele/sonoff/LWT
2017-07-21 20:44:28,520 INFO --- MqttFX ClientModel : messageArrived() added: message #5 to topic 'tele/sonoff/LWT'
2017-07-21 20:44:28,569 INFO --- MqttFX ClientModel : messageArrived() with topic: cmnd/sonoff/POWER
2017-07-21 20:44:28,570 INFO --- MqttFX ClientModel : messageArrived() added: message #6 to topic 'cmnd/sonoff/POWER'
no idea if that means my broker is alive and kicking…
mosquitto.conf seems empty?
[20:46:39] openhabian@openHABianPi:~$ cat /etc/mosquitto/mosquitto.conf
# Place your local configuration in /etc/mosquitto/conf.d/
#
# A full description of the configuration file is at
# /usr/share/doc/mosquitto/examples/mosquitto.conf.example
pid_file /var/run/mosquitto.pid
persistence true
persistence_location /var/lib/mosquitto/
log_dest file /var/log/mosquitto/mosquitto.log
include_dir /etc/mosquitto/conf.d
in /etc/mosquitto/conf.d/ are no files
clientID is unique for sure, i have only one MQTT device here (and got a random Client id wth MQTT.fx).
SOLVED!
i’m feeling like forrest gump here… “i am not a very smart man”
i configured openHAB with the same clientID as my sonoff device… it’s my first time with mqtt and i thought what openhab conf was supposed to look like 
Sorry for the trouble!