I use Firefox v.66 32bit.
1. Logged in YouTube as registered user.
2. Open new YouTube window in Firefox.
3. The window is opened, but after 3s the window refreshes many times after 0.5s again and again.
Found the solution, however need the normal bug repair:
1. When window refreshes, I turn off Add block, then press REFRESH button (in Add block) - then window stops refreshing.
2. I turn on Add block, then press REFRESH button.
3. Then press any video on page - shows "An error occurred. Please try again later (Play back ID:.......)"
4. You can wait ~5-10s when page refreshes automatically or you can press F5
5. Now video is working and Add Block also
6. If I open a new window with YouTube, the problem occurs on that window.