To understand why the frontend isn’t working, you need to stop the service:
sudo service mycodoflask stop
Then manually run the frontend to see any errors:
cd /opt/Mycodo/mycod && /opt/Mycodo/env/bin/python -m gunicorn \
--workers 1 \
--worker-class gthread \
--threads 2 \
--timeout 300 \
--pid /var/run/mycodoflask.pid \
--bind unix:/usr/local/mycodoflask.sock start_flask_ui:app