The fact that kdm worked for you could definitely mean that this has nothing to
do with X.Org.
rofl:~ # export XAUTHORITY=/run/sddm/{b8afb90f-46c8-4438-9a97-1bf82d128e7b\}
rofl:~ # export DISPLAY=:0.0
rofl:~ # xterm
Did you check the existence of this file:
/run/sddm/{b8afb90f-46c8-4438-9a97-1bf82d128e7b}?
If it does exist,
# export XAUTHORITY=/run/sddm/{b8afb90f-46c8-4438-9a97-1bf82d128e7b}
# xauth list
should give some output.