@markus_m2 In my case I am using one length of 5 metres of the Sonoff RL560 Sensor Extension Cable with the Sonoff DS18B20 Temp Sensor. The sensor is cable tied to the geyser brass (outlet) flange.
Today again it started registering the temp as it dropped below 40 C and then went blank as the temp went over 40 C. The temp is controlled by the geyser thermostat and I only use the THR320D Smart Switch to monitor the hot water temp while it is used.
Can anybody at Sonoff confirm whether the THR320D has this temperature range limitation of 40 deg C.