Diagnosis
This is usually a sign that the YouTube app's data on the Hub is corrupted, or there's a temporary account sync glitch between your Google account and the device.
How to Fix It
Start with the simplest fix. Say "Hey Google, sync my devices." Wait a minute. Then try YouTube again. If it's still broken, you need to clear the app's cache. Swipe down from the top for Settings, go to Apps, then See all apps. Find and select YouTube. On its info page, you'll see options to Force Stop and Clear Cache. Tap Clear Cache. Don't tap Clear Data unless you're okay with signing back in. After clearing the cache, go back and Force Stop the app, then open YouTube again.
If That Didn't Work
The next step is to check for a broader system update. Go to Settings > Device information. If an update is available, install it. If everything is up to date, do a power cycle: unplug the Hub for a full minute, then plug it back in. This often resets the network and app connections cleanly.
Heads Up
Sometimes this is a server-side issue with Google/YouTube that fixes itself in a few hours. If the problem only happens on one specific YouTube account, try removing and re-adding that account to the device in the Settings.