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surfacelane
The colors on the diagram matched my measurements with a multimeter, so that's not the problem.
Excellent.
A simple, helpful strategy is to swap drivers and/or motors. Connect the not working motor to another axis and look wether it works there. Replace the driver with one from another axis. Look wether the problem follows or not, this should give you an idea on which part doesn't work. Never forget to power down before disconnecting motors or stepper drivers. Running a controller with just one or two motors/drivers connected is perfectly fine, though.