See here the first rendered image (Cycles engine). You can notice that the background does not look very pixelated at all. This quality is fine:
However, in my renders (same Cycles engine, same render ... just some frames later), in subsequent frames, the background (and everything else) suddenly becomes very pixelated:
I hope this question can be answered. I have not found any other answers on this website, because this question does not relate to fireflies, and the render dimensions stay the change.
@Yohello1's comment -> Here are my output properties:
I render the video via my Mac's terminal: