Note: I’m on Digital Ocean and used one click app to install Discourse.
And this is what Digital Ocean Customer support said:
This appears to be directly impacted by the application your web server is trying to serve. We have a community article that can help provide guidance on general web server troubleshooting: How To Troubleshoot Common Site Issues on a Linux Server | DigitalOcean
If you use an application manager or proxy configuration ( php-fpm, passenger, gunicorn/unicorn, Tomcat, etc ) you will also want to try restarting the application itself if it is not running.
If the application has any debugging facility, that may be useful in narrowing down the source of the issue. Typically for application issues, the best resources will be the community, documentation and support forums for the application software itself. We encourage you also leverage our community as well: DigitalOcean Community | DigitalOcean
The system and application error and access logs may include additional details that could provide more insight on this as well. This guide will help you learn to work with common logs in typical log paths but your application configuration may use more specific locations: How To View and Configure Linux Logs on Ubuntu and Centos | DigitalOcean