The issue is solved. A friend of mine visited the place. He also had a talk with the owners of the last house. It is private property. No way through. The "kleines Tor zum Nationalpark" also was a private thing, nothing official, they built it themselves. I double-checked in official NRW-Atlas Liegenschaftskataster and there you can also see, that it is private property. The owners also said, that the path is overgrown etc. at this place. I changed everything accordingly.