The second prototype flies much better than the first one, I had no problems with the fixation of the motors anymore. But I fastened the screws on the center plates so strong, the plates bent slightly. I wasn't able to tune all the parameters, as I'm not sure how the copter will act on a open field (our garden is not as big as I thought).
Of course I crashed into one of the blue chairs after some time. Two of the legs are broken and one prop got three notches. Next time I will fly on a much emptier location. Nevertheless it was a success and I was able to practice flying.