It seems like the folder ./PIONEER/ is not present on your USB flash drive.
I use a similar setup (MacOs 10.13/RB5.0.1/NXS2 1.80) and no problems in this area.
Did you try to format the USB flashdrive in FAT32 and then export the playlists again to the drive?
Can you check if the folder ./PIONEER/ is present on your non functional USB flashdrive?