This mostly happens when you use youtube while being logged into a google account.
I fixed this by using firefox containers + addons.mozilla.org/en-US/…/switch-container/

• logout from google / clear cookies.
• switch to a new “google” container and login to see your subscrpitions
• if a video doesn’t load switch to decontained

also has the nice benefit of not messing up your reccomendations when clicking on random yt-links from other sites