OV = Overvoltage which means there's something up with the braking resistor or its circuits.
Look inside the drive cabinet for the brake resistor. It will be connected to the spindle drive with two big & two small wires. With the power off, disconnect the two big wires from the spindle drive to the resistor and check its resistance with a meter. It should be pretty low, say between 4 and 50 Ohms. (Let us know what you measured.) The skinny wires are a temperature sensor, so don't worry about them - you might not even have them. The brake resistor on my machine, with a Fanuc spindle drive, is a square metal can about 4" x 10" x 2" or so. I think you have a Yaskawa spindle drive as that's the only one that comes to mind that starts with a Y. (OK, Yansac too, but not in a machine this old).
If it measures in this range (it probably will), the resistor is OK and the problem will be in the spindle drive iteself. We will need to know more about the spindle drive.
How the spindle brake works is there's a transistor in the spindle drive that gets turned on to dump the excess power (voltage) from the motor slowing down into the brake resistor. If this transistor or the brake resistor fails the excess power doesn't have anywhere to go so the voltage builds up in the drive's DC bus which trips the drive's Overvoltage alarm.
I think the Yaskawa drives use a bunch of separate transistor modules so if one is bad it shouldn't be too bad to fix.
PS, thanks for starting a new thread!