Sounds like a possible controller problem. The heat would be coming from one or both of the pulse width modulated water control valves not being held closed. This is often the valves fault but both units are expensive so one would do best to test. The water valve should be tested both electrically (power and ground keep them closed) and functionally (clamp off the water pipes to verify inside cool temp).
Bosch Master, ASE Master, L1
33 years MB technician