I had this issue on OS 10.13.6.
I attempted to update the firmware on a brand new DDJ400.
Came back to the laptop and the update had failed for some reason.
The Channel levels LEDs were blinking and there was no way to reset it.
The only way to fix this was to plug the unit into another machine (OS 10.17.7), run the updater again until it reached 100%, and then the unit was no longer in update mode.