I googled this today because it keeps happening while I'm watching football. The only solution I found was to hard reset the box by unplugging it, waiting a bit, plugging it in, and starting it back up again (after checking that no guide/menu/etc. was open). I just waited because I didn't want to do that in the middle of the game.
While I was flipping channels, a program reminder that I had previously set for the game I was watching flashed on the screen...as if it were "behind" the main screen, unable to be dismissed. I then went to the screen for that team reminder and clicked "modify reminder" and clicked "confirm" without changing anything. Then I went back to watching the game with no screensaver.
The root cause seems to be an issue with reminders. A quick fix is to reconfirm one of your saved reminders. It'd be interesting to see if people without reminders also have this screensaver issue. @ComcastKenF
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