That's okay, my Dutch sucks, but Google Translate helps! :)
Okay, if I understand the situation, you had a problem where you lost your music, and then recovered the files from your USB drive. I'm not sure what happened in the middle part, but somehow you removed the files from the USB drive (?) but when you try and load them on the XDJ-RR, they show as 8306 ... is that correct?
If you happen to have all your music back in rekordbox, my recommendation would be to format the USB drive and re-export everything to the drive for use in the RR. It could be that somewhere in the process, you accidentally removed the contents from the USB drive, but left the database, so the RR finds the database and allows you to browse it, but then can't load the songs because they're missing.