An overflow occurs when an axis is expected to move (control is expecting pulses from the encoder) and none are seen. It thinks the motor is thus not moving. If axis is moving then the encoder circuit is at fault. If it is NOT moving, it could be a jammed axis, motor, wiring, drive, or loss of the command signal.
The BOSS 8 and 9 as well as the V2XTs and Explorers have a tendency of getting the quill jammed in the up position.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)