@DDPS; thanks. Yes!
Specifically, when you write:
that Multics-style indicator of path has to be watertight.
I’m only pushing that point because, as @BlackJack says, it’s vital to get paths just right.
If you had Playlists working before, did nothing - especially to the location and disposition of your actual music files - and now they fail, then I’ll be wrong.
But looking again at your paths - perhaps by loading a playlist into a spreadsheet, to ensure that each line has the required (format for) LF/CR, for instance - I do feel that’s a good place to start.
I too learnt the hard way; again, do follow that thread’s links.