I seem to have found the root of the problem I faced, with the cursor freezing on YouTube. I am going to write here the solution, so if anybody faces the same problem in the future, and comes searching, he will understand how to fix it. As I said I was having the problem only when I was on the 4K screen. So as time went by, I noticed that the cursor, as I was watching YouTube on fullscreen and it started to not work, when I paused the video, was also being distorted and when I right-clicked on the screen and the context menu appeared, the cursor started working normally again. Since the two screens are of different resolution, one is 1920X1080 and the other 3840x2160, I always had to go to Windows Display Settings and set "Change the size of text, apps and other Items" to 200% on the 4k screen in order for everything to look OK proportionally. This seems to not bide well will some apps and out of most of the browsers, the one affected is Opera, since the same problem never has happened to me, to this day, on Chrome, Firefox, Edge, Vivaldi etc...So what you have to do is, locate the "Opera.exe" file, right-click on it, go to "Properties", go to the "Compatibility" tab and select "Change high DPI Settings", from there tick the "Override high DPI scaling behavior" option and set "Scaling performed by:" to "System" (other options won't work right). Click on "Apply", close and open the browser(if you have it opened) and the problem is gone. This solution is quite handy on other apps also. For example "Microsoft Word 2010" was loosing its cursor, "VLC Player", "Plex Media Player", an older version of "μTorrent" were having different from each other text size issues and so on. So there you have it...
