Just speculating here, but my best guess is that it seems like there is a short occurring along the path or some type of corrosion on the circuit board–perhaps the extended use of the mixer builds up heat which creates a short in the faulty connection linked to the output of that specific channel. Since the signal from the CDJ is managing to be processed through the mixer as evident by the LED illuminating on the channel, you can at least isolate the issue. Are you able to hear the audio through the headphone cue?