Some time in the last few days, on the Bionic beta, SDDM fails to start.
Visible symptom is the Kubuntu logo pulsing for around a minute (normally replaced by graphical login screen in a few seconds). Then drops to text mode with single line at the top saying / is clean. I can change to the 2nd text console, and run "sddm" fine.
End of journalctl -b | grep sddm :
Apr 14 17:58:50 wopr.house systemd[1]: sddm.service: Main process exited, code=dumped, status=6/ABRT
Apr 14 17:58:50 wopr.house systemd[1]: sddm.service: Failed with result 'core-dump'.
Apr 14 17:58:51 wopr.house systemd[1]: sddm.service: Service hold-off time over, scheduling restart.
Apr 14 17:58:51 wopr.house systemd[1]: sddm.service: Scheduled restart job, restart counter is at 3.
Apr 14 17:58:51 wopr.house sddm[1046]: Initializing...
Apr 14 17:58:51 wopr.house sddm[1046]: Logind interface found
Apr 14 17:58:51 wopr.house sddm[1046]: Starting...
Apr 14 17:58:51 wopr.house sddm[1046]: Adding new display on vt 1 ...
Apr 14 17:58:51 wopr.house sddm[1046]: Loading theme configuration from ""
Apr 14 17:58:51 wopr.house sddm[1046]: Display server starting...
Apr 14 17:58:51 wopr.house sddm[1046]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{cb9a868b-c29d-464f-aa28-a9667d2ef02e} -background none -noreset -displayfd 18 -seat seat0 vt1
Apr 14 17:58:51 wopr.house sddm[1046]: Display server failed to start. Exiting
Apr 14 17:58:52 wopr.house systemd[1]: sddm.service: Main process exited, code=dumped, status=6/ABRT
Apr 14 17:58:52 wopr.house systemd[1]: sddm.service: Failed with result 'core-dump'.
Apr 14 17:58:53 wopr.house systemd[1]: sddm.service: Service hold-off time over, scheduling restart.
Apr 14 17:58:53 wopr.house systemd[1]: sddm.service: Scheduled restart job, restart counter is at 4.
Apr 14 17:58:53 wopr.house sddm[1185]: Initializing...
Apr 14 17:58:53 wopr.house sddm[1185]: Logind interface found
Apr 14 17:58:53 wopr.house sddm[1185]: Starting...
Apr 14 17:58:53 wopr.house sddm[1185]: Adding new display on vt 1 ...
Apr 14 17:58:53 wopr.house sddm[1185]: Loading theme configuration from ""
Apr 14 17:58:53 wopr.house sddm[1185]: Display server starting...
Apr 14 17:58:53 wopr.house sddm[1185]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{e33a9690-7283-46a7-a9d5-d9e25dcf7f3f} -background none -noreset -displayfd 18 -seat seat0 vt1
Apr 14 17:58:53 wopr.house sddm[1185]: Display server failed to start. Exiting
Apr 14 17:58:53 wopr.house systemd[1]: sddm.service: Main process exited, code=dumped, status=6/ABRT
Apr 14 17:58:53 wopr.house systemd[1]: sddm.service: Failed with result 'core-dump'.
Apr 14 17:58:54 wopr.house systemd[1]: sddm.service: Service hold-off time over, scheduling restart.
Apr 14 17:58:54 wopr.house systemd[1]: sddm.service: Scheduled restart job, restart counter is at 5.
Apr 14 17:58:54 wopr.house systemd[1]: sddm.service: Start request repeated too quickly.
Apr 14 17:58:54 wopr.house systemd[1]: sddm.service: Failed with result 'core-dump'.
Apr 14 18:00:11 wopr.house polkitd(authority=local)[782]: Operator of unix-process:1707:8740 successfully authenticated as unix-user:falken to gain TEMPORARY authorization for action org.freedesktop.systemd1.manage-units for system-bus-name::1.58 [systemctl start sddm] (owned by unix-user:falken)
Workaround : echo "service sddm start" >> /etc/rc.local
This indicates it's not a misconfiguration or hardward issue. SDDM is simply starting too early.