I am rather new to this, so sorry if the answer is obvious. I have searched a lot and not found an answer.
I have installed an AWS Machine Image with OpenFire (Apache), powered by Bitnami : AWS Marketplace: Openfire powered by Bitnami
I know, that the standard for the pubsub server would be a sub-domain like "pubsub.openfireserver.tld".
Thanks for any help!
Solved by using the standard sub domain. pubsub.[XMPP-SERVER-DOMAIN]