What causes this and how to avoid it in a render?
That is called banding, it is due to a lack of either high enough bit-depth or dithering. It could also be a combination of both. Try enabling dithering and see if it makes a difference.
it is often added when the image/ vid is compressed, sighhhh, one of the things I hate about UtUb
UtUb?
Throwing in a little film grain can help to reduce banding as well (it does something similar to dithering).
@lazycoder: UtUb = YouTube, now with banding (and in your sig!) 
@teoda: That’s not following model geometry by any chance? Just making sure it’s really due to the gradient.
Dithering did the trick, thank you!
Yay for Dithering! Can anyone find it in 2.5?