Sometimes YouTube asks people to sign in in order to watch videos, but it's not entirely clear why. Changing your IP address with a VPN might help, or if you are using a VPN already, try turning it off, or changing its location. Another option is to try using "Private" or "Incognito" mode in your browser (depending on what kind of browser you're using).

Unfortunately DU doesn't have any control over this -- it's entirely controlled by YouTube.

Wait a little while, utoob will let you watch them again. Sometimes it only lasts five minutes,other times much longer. Sometimes they will play after I watch something on utoob shorts and then I can watch them anywhere again. I think it's arbitrary.