Comment # 7 on bug 1229061 from Fabian Vogt

Sep 28 15:43:01 marcslaptop sddm-helper[3048]: Starting: "/etc/X11/xdm/Xsession
\"/usr/bin/startplasma-x11\""
Sep 28 15:43:01 marcslaptop sddm-helper[3089]: Adding cookie to
"/run/user/1000/xauth_WVbjKe"
Sep 28 15:43:01 marcslaptop sddm[1701]: Session started
Sep 28 15:43:01 marcslaptop sddm-helper[3048]: [PAM] Ended.

Hm, apparently this fails for some reason, but I can't tell why.

Just a wild guess: Does it work after uninstalling xf86-video-nouveau and
xf86-video-nv and rebooting? If wayland works, modesetting works, so xorg
should work without any HW specific drivers.