I am indeed getting a web server error:
upstream prematurely closed connection while reading response header from upstream
More specifically:
2018/12/14 12:13:34 [error] 18544#0: *41 upstream prematurely closed connection while
reading response header from upstream, client: 104.163.163.175, server: alexr.ca,
request: "GET /srv/gr_hello/color/query HTTP/1.1",
upstream: "http://127.0.0.1:3600/srv/gr_hello/color/query", host: "alexr.ca"
which might imply a backend.js timeout of some sort (authentication issue perhaps)…