A connection error usually means there is a server side error, can you check the logs/output.log when uploading an image in the composer.
certbot / nginx error - "Could not automatically find a matching server block"
Example here https://github.com/certbot/certbot/issues/5817
Got the same error, never got it before, using similar basic setups. Seems very common - Does anyone have any insight in how to get around or what to check?