Delete all your YouTube cookies reboot and try to play something from this thread otherwise this is the official explanation.
If you're seeing a "Sign in to confirm you're not a bot" error, it could be because a neighboring IP address is bombarding YouTube. In this case, YouTube may block the entire IP range you're in.
Here are some things you can try if you're seeing a YouTube ban block page:
- Remove any non-essential toolbars from your browser
- Run an antivirus scan on your device
- Contact YouTube support if you're sure your device is free of malware