This happened to me yesterday. I lost assist and the "WRN 601 SPD SENS" message scrolled across the screen. A bit later I got assistance back for a few minutes before the error message popped up again.
It appears the speed sensor cable is missing a zip tie and it is wedged between the swingarm and the motor. I'm guessing the wire wasn't hitting the tire while I was riding out of the trail do to suspension sag but It might have gotten snagged over some jumps we hit. I'll update this post when I know more.
Here a few pictures. The muddy tire is my bike. The clean tire is a my wife's bike for reference.
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