I was there today and wanted to take the path in question to access the Herbstbachtal and National Park trail further up. There were people sitting on the terrace of the building saying that the path doesn't exist anymore since the property changed from being a Gasthaus to a private residence.
I was a bit taken back because the National park info board/gate indicated on OSM made it seem like it should be a legit publicly accessible footpath, however I didn't want to press on the matter further.
Turning back, I overheard the comment "Ich will nicht dass jeder durch die Hecke geht", so it seems like a hedge was planted to block the path.
In any case, it would be good if you or another local person can check again and hopefully resolve the matter. I'm just a hiking tourist passing through. Regards