I am having a very similar problem. I built a BLFS system. Root has always been able to start xserver (startx) but not a normal user even before installing KDE (just using twm). Console login errors for normal users is usually either 'access denied' or 'no screens found' or something similar when it runs just fine for root. Still same problem after installing KDE. I can also boot into xserver, graphical.target, instead of console and still no normal user can login. Graphical login error for normal user login attempt is: "Could not start D-Bus. Can you Call qdbus?" I check status of dbus and it is 'active (running)'. After searching the web (all of it I think I have found many similar problems as mine going back several years and have yet to see any definitive solutions. Some have solved it by reinstalling x. I've build this entire system twice now with same problem. Mine is just a test box so I don't mind running x as root but would like to be able to login as regular user mainly because it is normal to do so and it is much easier to make a catastrophic mistake as root.
Maybe it is a bug with xorg server. Maybe a permissions issue. I would like to know.