Disabling it temporarily always fixes the problem, so I have assumed it is due to UBO blocking something which if YT sees has been blocked enough times for a specific visitor, they will prevent video from loading, but maybe my hypothesis is incorrect. This only happens when I’m watching a lot of YouTube videos one after the other.
I seem to recall that if you disable uBlock on a site, you have to reload for the change to take effect. Is it possible that the page reload and not disabling uBlock is doing it?