Suddenly "this library contains no local media content"

I have a new Apple TV 4K. Installation went perfect and everything was working OK.

But today, after playing some songs from my iTunes library, I got an error "This library contains no local media content". I can't find any music anymore.

I didn't change anything. I tried restarting the Apple TV and turning Homesharing off and on. But that didn't help. What is happening?