Did you change the DNS settings on your router or device? I had the same problem after installing pihole in my network, since by default it blocks

s.youtube.com and

video-stats.l.google.com

You coul do an nslookup on those subdomains, to see if you get an ip. Just open a command promt and run:

nslookup s.youtube.com and

nslookup video-stats.l.google.com

If you get one or more IPs in return, your problem is somewhere else

Are you logged in?

The only logical explanation I can make is you might have changed the password on your account you use to log-in to youtube and you need to re-add the account again in settings > accounts.