I would never ever disable the firewall on a public server…
It seems like you have issues with the nginx configuration…
Instead of trying to troubleshoot the issue (come on, it’s Saturday! ) I would recommend you [shamless self-promotion here] to use the installer script I created and released to the Zammad Community a few weeks ago.
This will do all the work for you and you will end up with a fully working Zammad server.