Well it is sitting here for me quite happliy and has done for several years.
Happy to keep going - the number of issue vice the number of installs is fairly low.
Really odd, can you post the output of sudo cat /etc/mosquitto/passwd?
Looking at the logs may help
journalctl -u mosquitto -f
[edit]
You can recreate the password file with
sudo mosquitto_passwd -c /etc/mosquitto/passwd emonpi
Then use the default password emonpimqtt2016
Restart the service with
sudo systemctl restart emoncms_mqtt.service
Ref - Can only connect with MQTT default credentials - #5 by Bigalboyo