Possibly it's time had come. Nothing last's forever. Per the old expression: "When you hear hoofbeats, think horses - not zebra's." Now if the next one melts.....
You've checked the wiring for shorts, especially the coil connectors. Might want to consider replacing all four coils just to save the trouble with the other three that probably aren't far behind. It did need a motor and a wiring harness.