Thank you Robert, it was the solution.
As I still have no final domain; i have simply used the internal hostname XXXX-vm-2.XXX.kit.edu
If I open it in the domain I receive only a NGINX page:
==================
Welcome to nginx!
If you see this page, the nginx web server is successfully installed and working. Further configuration is required.
For online documentation and support please refer to).
Commercial support is available at ).
Thank you for using nginx.
What ist the exact URL of a default Discourse installation?
I think If i finished the Config with e. g. SMTP, the Discourse should be already started, correct?
If you aren’t already familiar with hosting multiple domains and nginx reverse-proxy configuration it’s easier and faster to use another server for it.