Media file collapsed error when dragging files into CDJ's NXS2's

hi

I'm getting the media file collapsed message occasionally in my live sets. I'm connecting into a DJM NXS Nexus 2000 mixer with 2000 Nexus NXS2's in export mode using ethernet cables.

• It has happened twice both times near the end of my 5 hour set.
• Switching to usb sticks, rebooting rekordbox and the CDJ's fixed it and it didn't happen again that night(s).
• Both CDJ's are on the latest firmware
• I'm on a mac, High sierra, all up to date and rekordbox is too
• I run regular malware and virus checks and keep my rekordbox database as clean as possible
• It happens to both decks. current track playing isn't affected and continues to play luckily - no emergency loop
• Every track i try to load gives same error
• on reboot of decks and rekordbox same tracks then play fine with no errors

Any possible causes?

Thanks