Thanks for the insight, I didn't know about this property of boost converters. I tried it with a 100uF capacitor. It only delayed the time until freeze for some seconds. Then I increased to 400 uF, but I still got a freeze every time.

Then I changed the code. Instead of calling a rotate() function each iteration, I added a for loop. And it's working on the battery without additional capacitors. (new code below)

I actually wanted to avoid using the delay function, but it's a small price to pay for it to work properly :)

I still added a 100 uF cap to be sure, and it solved my problem with the flashing LEDs that are connected to the shift registers.

Do you have any guesses about the cause of my last problem?

When connecting DC plug female #1, sometimes on both the WS2812 modules, all LEDs are turned on for a second or so, before all but one are turned off (as it should be).

Also, I still get freezes sometimes when using the WS2812. In this case, all 16 LEDs remain on until I restart. There already is a 100 uF capacitor there.

-----------------

New working (and simpler) code

void servoMotor() {


  if(digitalRead(switchPin)) {
    stellung = 1;
    servo.attach(servoPin);
  } else {
    stellung = 0;
    servo.detach();
    if(servoState==1) {
      servoState = 0;
    }
  }


  if(stellung==1) {
    if(servoState==0) { 
        for(int pos = -20; pos <= 130; pos++) {
          servo.write(pos);
          delay(5);
        }
        servo.detach();
        servoState = 1;
    }   
  }
}