Just an educated guess as I don't know that heater at all but from the message it sounds like the water temperature sensor shorted. Sensors of any kind operate in some kind of a range and smart electronics can sense a short or an open and flag the error. My guess is it is a resistive element which require the simplest circuitry. Easy to sense failures too. Should be a simple fix once you find it. Would be located somewhere near the outlet of the heater. When you find it pull the wires off, most likely two on a connector, and see if the error changes to indicate an open. That's what I would try. Someone familiar with that heater can tell you more.
Al