I cannot say for sure what's causing your problem, I never had problems with the QR code scanner, but when I open it up, it seems that the quick settings QR code scanner is part of the "Google Play services" app. I'd make sure that that's updated for you - mine is currently on version 23.17.15 (190400-534251769) (231715044). Alternatively, you could fiddle around with things in app info, such as if you go to app info > top right 3 dots button > uninstall updates, then reinstall to see if that fixes your issue or clearing the cache/storage. Or maybe just updating to the latest would fix itIs ist possible to reset the QR-Code-Scanner Settings to get it work again?