What is the cause of the "502 Bad Gateway" after Ghost 1.8.7 update

I've experienced 502 issues with ghost behind nginx several times over a few years of running it. I'm not sure if the cause of mine today is the same as yours, but what I observed was that after a restart ghost had changed its port number to one different than what its nginx config was listening on.

I followed these directions from https://web.archive.org/web/20200807095031/https://www.danwalker.com/running-ghost-on-a-5-digital-ocean-vps/ which resolved it for me:

See which port ghost is running on:

sudo netstat -plotn

Check that it matches the proxy_pass in the nginx config file in /etc/nginx/sites-enabled.

In my case the port in the nginx config had incremented to 2369 while the actual node process was running on 2368. Changing the proxy_pass port back to 2368 in my ghost blog's nginx config file resolved the issue for me.