It's a fairly old NUC but still works well - Motherboard: Intel Corporation D34010WYK (CPU 1) 123 °C
Is this a problem and if so what may be the solution
Working in such high temperatures indicates failure in the cooling solution. Your NUC will be damaged.
Is the Cooling Blower working? Can you hear it? I suggest that you should clean the Thermal Solution including the Cooling Blower. Since all this is installed on board upper ( the invisible site), you need to remove board from the case in order to reach the blower. This video may help you (you may start watch from 8:10)
In this case, I believe that what has happened is that the thermal diode that is used to sense the temperature on the motherboard has failed. Seeing a static temperature like this is the usual sign of this issue. There is little that you can do about this type of failure. Since ensure that you are not having any fan speed control decisions made based upon this sensor. Using the Memory Temperature sensor in place of the Motherboard sensor offers a similar response curve.
Hope this helps,
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