I have run across this exact same problem today, I setup apache and haproxy for multiple sites, I confirmed the forum was working and up, I imported my myBB forum and everything was working. When I did a /launcher rebuild app, it broke the application and now i’m starting to get 502 bad gateway errors.
I checked df and free and I have more than enough space and resources (DO 2gb droplet), I did the installation myself and not the 1 click install.
Check firewall. Mine was a firewall issue firewall was blocking several processes.
For checking if a port is running fine do lsof -ni tcp:2222 where 2222 is port number.
If you are ok with deleting all firewall tables. In centos do this: