This sounds like an open circuit on the braking resistor. That is the thing inside the cage on top of the machine that gets hot.
When the spindle slows down or stops it does it by rgenerative braking and the controller dumps power onto the DC buss. When the DC buss voltage goes above a certain level the braking resistor, also called the regen resistor is brought into the circuit to bleed off excess voltage. I think there is a relay somewhere that is supposed to close.
If, for some reason, the circuit to the regen resistor does not close the bud voltage goes up and triggers and overvoltage alarm.
An open mind is a virtue...so long as all the common sense has not leaked out.