Offizieller Kommentar
You might be hitting a bug where Apple has renamed and/or deactivated an important XML file that Rekordbox relies on.
See this link: https://support.apple.com/en-us/HT201610
Follow the section titled About the iTunes Library.xml file
Finally, I believe there is a setting in Rekordbox where you can select your iTunes Library.xml file and I think I recall an issue where Apple recently renamed that file. So use Rekordbox and make sure to select the correct iTunes XML file - restart RB and try it all over again.
Good luck!