Ok, meanwhile I could solve this myself. Perhaps it helps others when I post my solution.
The problem seems to be browser dependent. I can not get Happanel to start with Firefox.
However with Chrome it works but only if you first login to OH3 main UI and then choose the alternative UIs on the right hand side:
Starting Happanel from there with Chrome then opens up the possibility to edit or load dashboards:
But even after having defined a dashboard and saving the Habpanel config it still did not show up in Firefox and also not in the OpenHab Android App:
The trick is to mark the Habpanel configuration with the small star in the Browser. Clicking on the star symbol right to the description of the configuration changes the filling of the star to a solid fill: