Synchronize iTunes playlists checkbox disabled in sync manager

As the title says, the "Synchronize iTunes playlists" checkbox is disabled for me in sync manager. I've correctly set the iTunes library XML location, and even though all the iTunes playlists appear in the iTunes column in sync manager, and all the playlists appear in the browser (under iTunes), the sync checkbox is still disabled.

I can obviously import them manually, but I'd like to be able to actually sync them. Is there something else I have to do to make sync enabled in sync manager?

EDIT: this is with rekordbox 4.0.5 and iTunes 12.3.2 on Windows 7 x64