I just wanted to share a recent experience. For the past few days, the YouTube apps on my Apple TV (4K) was misbehaving. When playing a video, I would get audio but only see a solid gray screen - no video content.
At first, I ignored it, assuming that it was some server glitch or that a recent app update has a bug and would soon be fixed. But that wasn’t the case - several days later, the problem persisted.
Then I remembered that you can force-quit apps on Apple TV using the application switcher (double-click the TV button). I did that for the YouTube app, restarted it, and it worked normally afterward.
Although it is obvious in hindsight, it wasn’t at the time. If you see a misbehaving app on a device (Apple TV, phone, Mac, .etc.) your first attempt for a fix should involve quitting and restarting the app.