Hey stradibarrius,
but have you read that in some cases connection was intermittent at times, like reported here and here, which means errors appear depending on movement/stretching of Z stepper cable - difficult to find when measuring at standstill. This can only be found by constantly measuring while moving/stretching the cable (while it is disconnected from controller and connected to multimeter constantly), and by doing this for all wires of Z stepper cable one by one (or to simplify, for one coil and then for the other i.e. measuring between a+ and a- and then between b+ and b-)