Edit, nevermind, it looks like it uses /usr/bin/X Did you make sure this is an issue specifically with SDDM, and not Xorg in general? do other DMs work, or can you start X with 'startx'?In any case it would be good idea to post logs from failed SDDM startup.I also had to downgrade (xorg-server, xorg-server-common and xf86-video-nouveau) but I'm running nouveau.Lightdm wouldn't start, nor would xfce4 with startxfce4, even with a new user. I would like to know if someone manages to run xorg-1.20 with sddm-0.17, in case is something else. SDDM likely needs an update (at least to a default configuration) to point to the location of the Xorg binary. I wanted to post it in case there is a bug/someone else has the same problem.
[ 744.043] (II) This device may have been added with another device file.Posting "me too" comments really doesn't help, especially when they're not necessarily related to the OP's issue.