Hi Valentin,
1000 words were fine but sometimes a picture tells you more
Please post a screenshot of your QGC mixer page.
What kind of ESC do you use? The settings needs to be altered if you're using SimonK FW based ESCs or KISS i.e.
MOT_ARM is too low for SImonK/KISS. I have used SimonK (hobby king Blue series ESC) with the values below:
MOT_ARM: 1050
MOT_START: 1065
Starting PWM: 1110
MOT_MAX: 1950
Values can vary for your setup, because of different motor/prop size. But they should give you an idea what to start with. You can check/alter them on the QGC Misc.Settings-Tab.
Check this first before you send the board to Jussi.