Recently I had a problem with discourse installation, as last time I installed discourse from scratch was version 1.3.1, and later just kept updating it. So some plugins I guess are no more supported by official discourse now. But I figured this out that one of the plugins I was using (slack) is no more supported after like 3-4 days of struggle and multiple reinstalls.
Just a thought or a suggestion that it would be nice if to remove such plugins or add a check during the plugin installation if such plugins were not supported. Because the error I was getting in the logs was “connection refused” thought it had to do something with the NGINX or firewall rules.