Common error resolution: How do I fix `Error: Already running on PID 1234`?

If you’re seeing this error:

Error: Already running on PID 1234 (or pid file '/tmp/gunicorn--dsmr_webinterface.pid' is stale)

Just delete the PID file and restart the webinterface:

sudo rm /tmp/gunicorn--dsmr_webinterface.pid
sudo supervisorctl restart dsmr_webinterface

Common error resolution: How do I fix Error: Already running on PID 1234?