With TW as OS and KDE desktop, when running SDDM as default-displaymanager
after boot login will only work with Plasma (Wayland) desktop.  Selecting X11
and logging in goes dark and then after a few seconds returns to login screen.

journal gives me:-
Sep 16 09:22:04 HP-Z640-1 sddm-helper[2134]: Starting X11 session: ""
"/usr/etc/X11/xdm/Xsession \"/usr/bin/startplasma-x11\""
Sep 16 09:22:04 HP-Z640-1 sddm[1851]: Session started false
Sep 16 09:22:04 HP-Z640-1 sddm-helper[2134]: [PAM] Closing session
Sep 16 09:22:04 HP-Z640-1 sddm-helper[2134]: [PAM] Ended.
Sep 16 09:22:04 HP-Z640-1 sddm[1851]: Auth: sddm-helper exited with 2 

and there is nothing in /etc/sddm.conf.d> or 
/.local/share/sddm> cat xorg-session.log (nothing here)