The solution that worked for me:
Fresh openhabian install on Rpi4. Installed mosquitto via the openhab-config. Mosquitto cleints failed to connect until I found this fix. Thanks.
It used to install problem and hassle free. This new issue wasted my weekend.
sudo systemctl stop mosquitto.service
sudo nano /etc/mosquitto/mosquitto.conf
Add:
listener 1883 0.0.0.0
allow_anonymous true
sudo systemctl start mosquitto.service