Updated 11:47 AM ET: At 5:36 AM, the Team YouTube Twitter account announced that it had resolved the issue and that everything is now back to normal. Carry on.
YouTube Premium and YouTube Music are very straightforward: you pay $12 or $10 a month to get ad-free videos and songs, offline downloads, and background playback. But things never are what they seem to be, as both services are being impacted by a bug that's breaking premium access for subscribers.
In the meantime, a quick workaround is to disconnect any Bluetooth or wired headphones and restart the app. Doing so seems to resolve the issue for the time being while YouTube works on a more permanent fix.