Hi
Sounds like a encoder problem or ONLY working with 2 phases. Usualy if it has danaher traction controller, machine try to work even without a phase.

Check encoder and encoder wiring, check also the current U, V, W with a ampmeter, should have same current in each phase (power Wire to traction motor)

Hi Lumberjack
It's an AC brushless motor

Check the drive motor brushes.