Problem solved! Or at least I think so. I haven’t had time to work on it today, but last night I gave the MK scarf another shot. I think it was the yarn feeder in the tension arm. It’s got a feeder like I haven’t seen on any other knitting machine, and the yarn was getting twisted around a clip somehow.
Since I saw it happen and corrected it, I haven’t had any problems. Hurrah! I fiddled with it for a few minutes last night and I’m already well on my way to finishing the scarf. Happy times again in MK land.