Appletv+ Not Working

On the Apple TV app in Xfinity, go over to the Settings on top and scroll down to the "Reset Apple TV App", then try signing in again.

I saw you rebooted the Xfinity box, have you tried restarting the Xfinity WiFi router as well?

If rebooting the router does not help, the Xfinity site also has an option called "System Refresh" in the TV troubleshooting section that may be more involved than a simple reboot of the device.

Other users on the Xfinity support site that had the same error said it resolved by itself, but since it has happened for over a week, that is unacceptable. Another user who had multiple TV boxes said it worked on one, but not the other. They then switched the TV boxes and said the problem followed the box to the other TV that it was connected to. Their solution was to have the box replaced. I would not expect it to be a hardware error, since it sounds more like a network connection or software error that should be fixed by a reset, but it seemed to work for them.