AppleTV won't play "Video Not Available"

For several weeks, AppleTV on our Roku will not actually play anything. (Note that it previously worked fine; this started randomly.)

When we try to watch something, we get the spinner for a little bit; then very, very briefly (less than a second), we see the video with the progress bar; and then we get this message:

We have a Roku Streaming Stick model 3810X. It's running version 11.5.0 build 4313-50.

Here are all the things I've already tried (though not necessarily in this order):

1) Reset the Roku.

2) Make sure the Roku is on the latest software version.

3) Remove the AppleTV app from the Roku and reinstall it.

4) Remove the AppleTV app from the Roku, reset the Roku, and reinstall the AppleTV app.

5) Unplug the Roku from the TV and plug it back in.

6) Unplug the Roku from the TV, unplug the router, wait 20 seconds, plug the router back in, wait for it to come back online, plug the Roku back in, then perform Network connection reset, and then set up the connection again.

Nothing has changed after any of these attempts. I don't know whether the problem is with the Roku or with the app, but I'd appreciate some help. (If I can, I will post the same message in Apple's forums.)

Tamar