I fixed the LEDs flickering by adjusting the Grabber cropping, so that there were no margins:
The margins had visual artifacts and noise, hence the flicker. I have used "Take grabber screenshot" to see where are my margins.
My grabber had no option to switch to NTSC, seems to be fully automatic. Id does work only while set to NTSC in Hyperion.
Grabber: LogiLink
LED strip: APA102.