About 3500KM in, and too meticulous when cleaning, and an instance of crossing the river (twice), water found its way inside the motor and eats up the bearing, and leaves rust in its wake.
Solution: open the hub motor, change the bearings, clean every surface, re-grease the planetary gears, and hope for the best.
Though it did not return to a factory-sounding brand new motor, at least the ride is smoother now.