Comment # 3 on bug 987487 from Fabian Vogt

I doubt that this is an sddm issue, this is either X being unable to start or
ksmserver crashing for some reason. Try a different window manager, like IceWM.
If that works, try plasma again and upload ~/.xsession-errors-:0. If it
doesn't, attach /var/log/Xorg.0.log.