Why does my phone keep on saying "Unfortunately, System UI has stopped"?

[AC Question]

I have an LG G3 (lollipop 5.0.1) and whenever I clear all of the recent apps that I used and tap on the recent apps button again (the square button on the navigation bar of the lollipop os), the message "unfortunately, system UI has stopped working" pops out. I factory reset my phone three days ago because the same message kept on popping a week ago so I figured resetting it would do the trick, but yeah, apparently not.

 

[Winston P]

Have you installed anything since then? Try resetting the phone through the recovery menu. If the problem persists, I'd recommend flashing a new ROM onto the device, or just take it to LG to get them to do it for you.