Video grabber flickers with artefacts

[Jejebond]

Hi,

I have a strange issue since i try to go 4k 120hz HDR with a new optical HDMI cable (RUIPRO, 20 meters cable) between my LG C9 and my PC.

Pxl.20241020.064656411.Ts.mp4

Here is what the grabber shows :

But the TV is showing clear image, 4k 120hz HDR without issue.

I've tried Vsync, lowering resolution, lowering FPS in Picap, same results.
If i try to set my PC back to 1080p 120hz, it won't work every time. Could it be VRR then ? some other tech interfering ? A setting on my LG C9 ?

I suspect going 120hz gives me this issue every time, even in 1080P.

Is this a know issue ? Any ideas ?

Thanks a lot