Do you use unencrypted login at port 1883?
The problem encounters because of the default setting of mosquitto v2.0+. At default settings, nobody (devices & openhab) was able to connect to the mqtt server after i reinstalled openhabian.
Try to add these two lines (first line is really needed):
sudo nano /etc/mosquitto/conf.d/default.conf
listener 1883
allow_anonymous true
Afterwards restart mosquitto.