Temperature Sensing and board heat
It turns out that with a cable length of about 20cm I am able to get believable results. And I am able to confirm these measurements with a weather station right besides the sensor. It is to 0.05 degC exactly the same. This is what initially I expected these measurements to be.

I will leave this question here for reference for anyone wondering what a huge influence the circuit-board heat can have even with moderately long cables of about 7cm.