YouTube is broken for me, pic related.
I've tried deleting cache and cookies, but it's still the same. Also, it works well in other browsers, but Vivaldi is my main browser and it annoys me to have to open other browsers only to watch a video.
disable every extension in vivaldi and see if it works
if it does, enable the extensions one by one and check which extension causes this
it could also be possible that two extensions are not compatible with each other