Thanks Marcus for your prompt help. I suffered the same issue with a new install of the openhabian image on RPi3b.
Following your instructions, repeated below for those that need it too, everything works as it should, no errors:
"mkdir -p /var/log/mosquitto /opt/zram/log.bind/mosquitto
chown mosquitto /var/log/mosquitto /opt/zram/log.bind/mosquitto
systemctl start mosquitto.service
Create /lib/systemd/system/mosquitto.service.d/override.conf to contain
Thanks for your help.