I stumled upon this from the Openbazaar documentation and applied the knowledge.
So I modified the openbazaar config as follows:
I changed to tor only mode:
I also added the IP of the Gateway (10.152.152.10) and the Control Port 9051. In the torrc file on Whonix Gateway no paswword was specified so I did not change that.
In the Openbazaar client I checked the box “Use Tor” and entered 10.152.152.10:9050 in the server configuration.
However, unfortunately it is still impossible to connect to the server with this more advanced configuration.
Further hints/ideas/help are still needed and appreciated to get this thing running on Whonix.