Two(!) CDJ-2000 NXS2 that would not read USB

This happened yesterday. At a club both CDJ-2000 NXS2 players all of a sudden would not read anyone's USB stick anymore. Power cycling the players made no difference. The only way we could get the player to sometimes read a USB stick, was by turning the player off, insert the stick, then turn the player on again. After doing this a few times, it would read the USB stick. Like I said, this wasn't one player, but both. And not just my USB sticks, but all DJs USB sticks.

It's not the sticks that are at fault. When I got home, my own CDJ-2000 NXS2 players read the stick just fine.

What could cause this?