thanks for your promt replies, I really appreciate it!
Below two pictures. On the display, the button in the back to the right is going up and down at irregular intervalls without me touching the real button.
The legs of the button are not bent sideways or so and apper to fit nicely into the board. I tried exchanging the button to another one to make sure it is not broken. I also tried the same buld-up at another place of the breadboard to make sure there is not a wiring defect inside the board.
Felix / LenaKonst