Thanks sooo much for the detailed, time intense, answer Stefan, that was very enlightening.
I already suspected that there could be a problem with the delay, but felt obligated to leave it in because it was suggested in every of the DHT11 examples, because the refresh rate of the Sensor is pretty low. And as i read thru the debugged version of my sketch that was suggested by you I thought that could may cause trouble, but that part works just fine.
However the set Value (solltemperatur) still doesn't rises (or lowers) when I turn on the rotary. I thought it's probably toasted after all that trial and error so I tested it again with the KY-040 example program and the modified version (sirial monitor version) of my program but both times I was able get a result in the serial monitor, so it seems to be fine.
Unfortunately I have an exam tomorrow so I don't have enough time for a deep dive into debugging right now. I will try to figure it out tomorrow with a little more time and a little more focused state of mind.
Thanks again
Mads