The below issue occurred in rekordbox Ver.4.2.3

Please update rekordbox to this version (Ver.4.2.4)

Please note: When you sync playlists which were not synced in Ver.4.2.3, firstly please untick the unsynced playlists and click the Sync button (the arrow icon). Then, tick the unsynced playlists again and click the button to sync them.

Change

• Fixed
  • Sync Manager did not sync playlists.

Download rekordbox here.