Dude! Gothic Gardens works! Right of the bat, first difference is, it is only called Walkway here (instead of Greenhouse Walkway). Could the naming be the problem? Thanks for the great suggestion and I'm going to investigate a bit further now.
Edit: Checking the itp file with the gff editor, there is a note saying, there is a 16 letter limit for CResRefs, which is exceeded in case of Greenhouse Walkway. It also says spaces are not allowed, but that doesn't seem to actually be true, since the Forest Clearing terrain works just fine with one.
Edit2: Success! Renaming the crosser in the set file to GH Walkway, replacing all references to it in the set file and creating the according itp file worked. All three of you were pointing in the right direction, thank you, guys!
I guess then, a tile name should not be longer than 16 characters.