Thanks for helping! I can access the log and get the following:
[Mon Oct 16 16:59:52.821276 2017] [proxy:error] [pid 6193] [client 90.88.142.181:51638] AH00898: DNS lookup failure for: 127.0.0.1:8050_dash-layout returned by /_dash-layout, referer: http://sub.example.com
It is definitely a problem on the proxy/server side rather than a Dash one, I’ll have a look on the material you provide on Flask.