WS2812b/Sk6812 flickering on camera
From the comments to Justme's answer:


  This is what I don't understand: WS2812B data sheet says the data rate is 800 kb/s and one RGB data package is 24 bits long. That means one LED will get new RGB values with 33 kHz.


Almost. There's a reset pulse to be taken into account too, but applications with a single addressable LED would be rare as a cheap common anode RGB LED could be used instead. For longer chains of LEDs the maximum update rate would be 33 kHz / # of LEDs.


  This would be then way more than the 400 Hz for the physical LED output current.


The two rates would, most likely, be independent. The data will be latched when it's received and applied to the PWM controller at the start of the next PWM cycle. From the point of view of the camera the data rate will be irrelevant. 


  A PWM rate of 400 Hz means that the shutter speed of the filming camera must be lower than 1/400, correct?


No, higher than 1/400 (2.5 ms) but I think that a lower number in the denominator is what you meant.

Be careful with this. If you match the PWM duty cycle exactly then it would be fine but at slightly longer cycles there would be variation.

       ________          _________           _________          _________
PWM __|        |________|         |_________|         |________|         |______
        ____________________             ____________________
CAMERA_|                    |___________|                    |__________________
         1.5 PWM cycles                      1 PWM cycle


Figure 2. Variations in light captured by the camera depending where in the PWM cycle the shutter is triggered.

To avoid this problem you would need to ensure that the exposure time was long enough to include at least, say, five PWM cycles.