PS. There is no sign of loss of power when the axis is not moving when testing manually the motor shaft.
PPS. This seems more and more weird when I test a little further: When the y axis is kept put, it's power is OK. When I start and increase the speed of the spindle the power of the y axis shaft starts to diminish. When I stop the spindle the power returns back and no error in VSD-A and it locks back to position. The same power loss does however not happen to X and Z axis while all 3 VSD-As are identically connected and been working fine so far. The spindle's VFD drive is outside the cabinet and there are no VFD related power lines going near any of the VSD-As. The only thing they share in common with the spindle is the CNC4PC C11 BOB. Can anyone make sense of what could be happening here?