Comentário oficial
It seems there are a number of users having issues with 4.2.0. The downgrade to 4.1.2 is recommended at this time, you will not lose your collection.
Additionally, you can revert to a previous version of your database through an automatically created backup in the rekordbox folder:
Win 7/8/10: C:\Users\yourusername\AppData\Roaming\Pioneer\rekordbox
OSX: /Users/yourusername/Library/Pioneer/rekordbox
Rename datafile.edb to datafile.edb.old
Rename datafile.backup.edb to datafile.edb (removing .backup)
rekordbox automatically creates a backup each time you launch the software; if you have opened / closed v4.2.0 more than 3 times you may not be able to recover your database in this manner. Please check the date / time of the backup files in order to choose the most appropriate version.