I was searching for the similar questions, it seems that problem may be caused by a communication issue between the reverse proxy and the internal server who is listening on port 8080, this premise can be endorsed by the fact that Kobotoolbox keep showing Waiting for environment to be ready. It can take a few minutes and never outputs the successful startup message.
Would you mind to share your kobo-install .run.conf (hide any secret credentials).
Usually, when extra_hosts is commented is because you choose the option with the private DNS on your local network. So the domain names resolution is happening thanks to the DNS server.