Warning Icon in FullyKiosk's Screen Saver

I don’t know when it started, but my wall panel (Fire tablet) running FullyKiosk is showing a small warning icon in the upper left corner of the screen saver. I’ve gone through all the options and menus, including on the Fire tablet itself, and I cannot see anything wrong.

Where could it come from?

Turns out the problem was with the Wall Panel HACS extension. I turned off Wall Panel’s screen saver by setting “idle_time” to 0 and am letting Fully Kiosk’s do the screen saving.