From the Netherlands (eu), Firefox on Linux - youtube.com shows modal about cookies, "reject all"... Videos just work.

I suppose you just ended up on wrong side of google's captcha. Which basically relies on ... Everything.

...Good luck

I briefly had that in the past, but it went away within a day or so. I may have also reset my YouTube/Google cookies. I think sometimes the site cookies get into a weird state, but I believe it can also be that your IP address was flagged for some reason. In the former case, switching browser profiles or deleting cookies should fix the problem, but in the latter case, you'd probably need a VPN or something, and unfortunately some VPNs could also just be blocked outright. You could try Tor as well, but it also won't reliably get you through without logging in, although sometimes just rerolling the dice with the Tor circuit can work.

There could also be other workarounds like VPSes, but TBH I don't know how reliably they can access the site, if at all.