Please advise is this gearbox was new or used unit. It seen the rubber block were deteriorated by overheating and it could be caused by lack of ventilation ( between front housing and input coupling), torsional vibration or misalignment.
Can you provide the engine model, photo on the gearbox and kindly find out with customer any abnormality after they replaced with new rubber blocks.
Meantime, did your service department had on board to inspect and check the gearbox when customer reported the failure?