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Please try updating to Sierra 10.12.6 and iTunes 12.6.2.20? It seems to work for me:

I have a lot of playlist folders, that leads into a lots of playlists, so that i can have my iTunes library organised.
In the past it used to work perfectly with rekordbox, and i just had to open rekordbox and it will start to sync up with my organised iTunes library.
But now when i have updated the programs a few it dosen't work anymore.
The playlists will simply not show up, with hitting the little + on the side of the playlist folders.
I can open the folders that are not in the playlist folders - for instance the last played folder. But it wont let me open the playlist folders.
I run iTunes 12.6 won't show in Rekordbox 4.5, on a Mac OSX Sierra 10.12.4.
Have someone tried anything similar? And do any have a found a solution for this yet?
Please try updating to Sierra 10.12.6 and iTunes 12.6.2.20? It seems to work for me:

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I have the same problem on 4.5 w/ osx 10.9.5
Probably an issue with itunes
@Luke > Which version of iTunes are you running?
Also 12.6. I was able to fix it though by running the sync manager. Its something I never had to do before but now that I know its fine.
Your iTunes library file should load the playlists without running the sync; you also want to make sure the "share with other applications" box is ticked within the iTunes preferences.
Yeah that's how it always worked before, including playlists in folders. I tried reverting back to 4.2.5 and it has the same issue. It will show new playlists if they're not in folders without hitting the sync button but if they're in a folder I have to use sync.
And yes the xml sharing preference in itunes is clicked on.
Not a huge deal, worth hitting sync to make sure waveforms are analyzed/beat-gridded anyways.