As you expected, it was the HDMI cable. I thought that I was already using a 2.1 certified cable but it turned out that this wasn’t the case.
I hadn’t realized that there’s a good reason why other video’s with a much higher bitrate were playing fine: the frame rates were much lower (24 fps). So video’s with 4K at 60fps have a much higher bitrate over the HDMI cable exposing it’s true quality.
I’ll do some further testing but for now everything seems to work fine.
thanks!