@Quirin > The reason those were missed was you piggy-back posted on another thread. Starting a new thread, regardless of how related your problem is, is the beset way to get assistance.
What is the make / model / USB-type of the flash drive you're using? When you originally formatted them, did you format them TWICE using a utility other than the Windows built-in format function? What filesystem did you format the drive to (eg. FAT32, HFS+)?
Which version of rekordbox are you exporting with?
The issue in the first video is mostly to do with the read-speed of your device along with the limited memory of the CDJ-850 (which is why the error doesn't happen with higher-model players). The second video is a known issue, but is a standard function of the player when it attempts to load the track previous to the one you are scratching from the 0:00 time.