What's up with the video?

So I already have that with 2 notebooks that always occurs briefly and disappears again.

This almost always comes only briefly:

I often had a mistake in the video but it is not because of your note book

Maybe it's Firefox.

That should be because of your notebook, but it could also be something else, but I had it with another device, I just used another browser, then it was gone, but it can also be due to the quality of the video

Try another browser

No just a mistake that always appears in the same second, a mistake by cutter or something else

https://www.youtube.com/....com/html5

Activate the html5 player here. If it is already active, deactivate it. Maybe that will solve the problem