What they are telling you is: “No, you need to buy something.”
Nuendo 8/10 (stock) will not work with Studio Pass. You have to buy VST Connect Pro. (or register for a trial license if there’s a trial available)
Studio Pass works with VST Connect Pro, and does not work with SE.
Nuendo comes with SE, and does not come with Pro. End of story, Gregor. 
If you have a menu item for Pro in Nuendo 8, that could be because it came with the Nuendo 8 installation to facilitate demoing VST Connect Pro, or you installed it. Either way, you need a license, trial or full retail,to use it.