Reply to S4 not responding "ON" in loop length display

hey guys.
i'm chilling with my bud, spinning a little, when all of a sudden the S4 doesnt respond anymore and the loop length display shows "ON". we fiddled a little bit with the keylock (shift + play) but nothing special.

restarting the S4 didnt do the trick, restarting traktor did.

i tried searching the problem, but google didnt come up with any good answer.

what kind of error message was this? how can we prevent this from happening? and are there any other solutions than restarting traktor?
(dont want run into this in a live situation)

thanks a lot