Z axis motor heats and loses zero point.

Hello everybody,

I have the stepcraft 840 and I am using it for some time. Now when I am using the z-axis it loses it zero point. And it goes up by 2mm approx.
However I noted that my z-axis motor is getting hotter than the other motors and it sounds like there is still field excitation active when the motor is not in operation.
All terminals are ok and no damaged on the circuit board.

Does anybody know what the issue can be?

Thanks!