rekordbox : CDJ error E-8302 cannot play track (3620)

From experience, most people get this because they move or alter songs on the Export USB after it was created by rekordbox.  A USB key exported by rekordbox has a TOC (table of content) and the /CONTENT folder, which hold the songs.  If you remove songs from this /CONTENT folder and load it again, you will still see the removed songs in the track list (because it's in the TOC), but you'll get error E-8302.

There are probably more possible causes for this error (corrupt audio file, unsupported characters in audio file name, ...) but this is the most common one.  If you think that is the case, format your USB key and export from rekordbox again.