Okay, I encounter some strange behaviour now. I used an arduino uno instead, with the same sketch/script (but bootloader for Uno of course) and now the leds are always off. I checked and the voltage of the data pin is always same as ground. So I declared 'pinMode(3, OUTPUT);' in the 'setup()', but without results. I checked if the script would run by using the builtin led. And the strange thing is, it doesn't. So when I remove the 5V pin from the neopixel, the arduino starts blinking 3 times (I added 2 times in my script, but this is from startup I read somewhere). But if I add power to the breadboard with neopixel, the arduino doesn't do this. Looks like the arduino goes in safe mode, even while the voltage over the arduino is 5V and no other pins are connected.
Does anyone maybe now know what I am doing wrong?