CDJ-850 and Rekordbox 'Library is not Responding'

I use USB sticks to play on my Pioneer CDJ-850s and up to this day, this always worked, until now. On every USB stick I use I get the error message: Library not Responding. I wonder if this has anything to do with me using Rekordbox on the USB sticks, I use version 4.2.1. 

Suddenly both of my CJS cannot read any of my USB sticks, not even ones just containing MP3s whereas before, the CDJs could. The issue occurs on BOTH of my CDJs, not just one, even though they are not connected. This is why I suspect this is a Rekordbox problem. 

Please help! Without this, the cd players are useless.