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Jelby-John
Bad connection along the cable is very unlikely given that it affects a stepper which doesn't move at all (and has a short, straight cable run).
It doesn't matter whether the motor moves frequently or not: breaking the connection while a motor is energised is probably the most common way of damaging a driver. It is easy to get a bad crimp connection where the connector is crimped to the end of the cable, especially if it wasn't made with exactly the right crimp tool.