But how is it possible that I have actually measure this apparent movement of 15 degrees per hour? I measure this myself many times? How can that be possible?

It is possible because:

1) You do not understand the geometry of the positions of the earth and sun and how the sun moves across the sky with respect to the local earth coordinate system.

I do understand simple geometry. I studied mathematics and physics

2) You have measured incorrectly, and with insufficient error bars.

Could be a possibility, but with many repeated measurements it is easy to see that the deviation is less than 100%. That is sufficient to rule out flat earth

3) You have not read and comprehended the explanations given in the previous (now this) topic.

Irrelevant. It is about the observation of the apparent motion of the sun. It is just about an observation. Independent of any explanation

4) You have convinced yourself that a 360 degree earth rotation in 24 hours is equivalent to an apparent angular motion of 15 degrees/hour, and that has biased you into "fudging" your measurements to give the correct answer.

Incorrect. I am interested in reality, the truth, science, knowledge.

Even using predictions (like the NOAA Solar Calculator) confirm that the angular position of the sun does not agree with a 15 degree/hour apparent angular motion rate.

Irrelevant. If earth were flat the angular motion must be MUCH smaller close to a sunset/sunrise. An approximate measurement is enough.