Since last round of KDE updates in MGA5, I have the same problem in two systems : kdm fails to start. One is i586, the other is x86_64. My other 7 systems have no problem with this update.

It appears that after boot, I only get a dark console with no text or error message. I can only give this debug info :

systemctl status prefdm.service 
● prefdm.service - Display Manager
   Loaded: loaded (/usr/lib/systemd/system/prefdm.service; static)
   Active: active (running) since ven. 2017-09-01 22:37:13 CEST; 2min 54s ago
 Main PID: 778 (kdm)
   CGroup: /system.slice/prefdm.service
           ├─778 /usr/bin/kdm -nodaemon
           └─839 /etc/X11/X :0 vt7 -auth /var/run/xauth/A:0-PI8iqb

sept. 01 22:37:13 adear32.local kdm[778]: plymouth is running
sept. 01 22:37:13 adear32.local kdm[778]: plymouth is running
sept. 01 22:37:14 adear32.local kdm[778]: plymouth is NOT running
sept. 01 22:39:13 adear32.local kdm[778]: X server startup timeout, terminating

Doing "systemctl restart prefdm.service" seems to workaround the bug.