It works with when I start with

node app

not with

./nodebb start

though.

Hello. I have not made any significant modifications.
I am using http-proxy to reverse-proxy traffic to the forum so that I can run another app on the same domain.
The proxying and forum was working well and then unexpectedly this happened when I start it:

NodeBB v1.0.0 Copyright (C) 2013-2014 NodeBB Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it under certain conditions.
For the full license, please visit: http://www.gnu.org/copyleft/gpl.html

Clustering enabled: Spinning up 1 process(es).

[cluster] Child Process (21588) has exited (code: 0, signal: null)