Looks like we have a fix in for this:
https://old.reddit.com/r/brave_browser/comments/1hinfh3/youtube_now_suddenly_blocks_me_for_using_brave/m31kcmw/
In brave://components force an update of Brave Ad Block Updater too Version: 1.0.9653 or better
Then refresh Youtube.com tabs (important). Let me know if it helps
Ensure no other adblocker extension is enabled.