Here, "/etc/xdg/kcm-about-distrorc" had this data:
[General]
LogoPath=/usr/share/icons/artix/logo.svg
Website=https://artixlinux.org
Version=x86_64
Variant=Rolling Release / KDE Plasma
the same, that the file had after installing only "artix-qt-presets".
I searched for this package:
sudo pacman -Ss artix-qt-presets
galaxy/artix-qt-presets 20200506-1 (artix-branding) [Installiert]
Artix Qt presets, for KDE Plasma and LXQt.
and decided to remove it, as I don't use KDE or LXQt.
(I have FVWM, LXDE, Openbox, Sway and XFCE installed)
After removing "artix-qt-presets" the system update worked. And the file contained only:
[General]
LogoPath=/usr/share/icons/artix/logo.svg
I hope it is okay like this.