Can you change the log level please
nano /var/www/emoncms/settings.php
and change $log_level to 1 (INFO).
I’m not sure this will help as my system does not seem to be logging correctly (may be related to path changes see Is Emoncms logging broken).
Set the autorefresh to on (in the admin page) and see what the log produces.
[edit]
You need to restart the MQTT service for it to pick up the change in log level
sudo systemctl restart emoncms_mqtt.service