This bug report is not for NodeBB itself but for your instance of it at "community.nodebb.org".
So nginx is setup wrong serving a "502 Bad Gateway" text error page instead of a custom page displaying something like: "We will be back soon".
This isn't necessarily a bug, just something that hasn't been configured in nginx.
Hahah you accessed NodeBB right between restarts, maybe
Yes, we don't have an error_page value set. We really should...