Comment # 1 on bug 1141391 from Felix Miata

Fedora 29 has the same problem except with a much more recent KDM, possibly a
clue to fault not in either of the KDMs:
# inxi -S
System:    Host: big31 Kernel: 5.1.16-200.fc29.x86_64 x86_64 bits: 64 Console:
tty 3 Distro: Fedora release 29 (Twenty Nine) 
# rpm -qa | grep kdm
kdm-4.11.22-23.fc29.x86_64
kdm-settings-4.11.22-23.fc29.noarch
# systemctl status kdm
��� kdm.service - The KDE login manager
   Loaded: loaded (/usr/lib/systemd/system/kdm.service; enabled; vendor preset:
enabled)
   Active: active (running) since Mon 2019-07-15 00:37:26 EDT; 3min 39s ago
 Main PID: 560 (kdm)
    Tasks: 1 (limit: 4704)
   Memory: 66.1M
   CGroup: /system.slice/kdm.service
           ������560 /usr/bin/kdm vt1

Jul 15 00:37:26 big31 systemd[1]: Started The KDE login manager.
Jul 15 00:37:28 big31 kdm[560]: plymouth is NOT running
Jul 15 00:37:58 big31 kdm[560]: X server startup timeout, terminating
Jul 15 00:38:03 big31 kdm[560]: X server for display :0 cannot be started,
session disabled