Another monitor-related bug like this one https://forums.opensuse.org/showthread.php/512997-Plasma-5-KDE-Switching-monitor-off-will-cause-some-KDE-malfunctions, this time sddm is to blame.
At startup, when the greeter is displayed, if the monitor is switched off and then on, sddm-greeter exits for no apparent reason, making it impossible to log on to the graphical session.
Logs:
2016-01-28T23:11:19.980147+01:00 kenshiro sddm[1091]: Running display setup script "/etc/X11/xdm/Xsetup"
2016-01-28T23:11:19.993288+01:00 kenshiro sddm[1091]: Display server started.
2016-01-28T23:11:19.993497+01:00 kenshiro sddm[1091]: Socket server starting...
2016-01-28T23:11:19.993639+01:00 kenshiro sddm[1091]: Socket server started.
2016-01-28T23:11:19.993885+01:00 kenshiro sddm[1091]: Greeter starting...
2016-01-28T23:11:19.994039+01:00 kenshiro sddm[1091]: Adding cookie to "/run/sddm/{5ab51fe3-b0e9-41b7-b2ed-294821cc0322}"
2016-01-28T23:11:19.998523+01:00 kenshiro sddm-helper[1653]: [PAM] Starting...
2016-01-28T23:11:19.998753+01:00 kenshiro sddm-helper[1653]: [PAM] Authenticating...
2016-01-28T23:11:19.998916+01:00 kenshiro sddm-helper[1653]: [PAM] returning.
2016-01-28T23:11:19.999052+01:00 kenshiro sddm-helper: pam_unix(sddm-greeter:session): session opened for user sddm by (uid=0)
2016-01-28T23:11:20.014407+01:00 kenshiro sddm[1091]: Greeter session started successfully
2016-01-28T23:11:20.027505+01:00 kenshiro sddm-greeter[1655]: Reading from "/usr/share/xsessions/enlightenment.desktop"
2016-01-28T23:11:20.027786+01:00 kenshiro sddm-greeter[1655]: Reading from "/usr/share/xsessions/gnome.desktop"
2016-01-28T23:11:20.027936+01:00 kenshiro sddm-greeter[1655]: Reading from "/usr/share/xsessions/icewm.desktop"
2016-01-28T23:11:20.028071+01:00 kenshiro sddm-greeter[1655]: Reading from "/usr/share/xsessions/kde-plasma.desktop"
2016-01-28T23:11:20.028204+01:00 kenshiro sddm-greeter[1655]: Reading from "/usr/share/xsessions/plasma5.desktop"
2016-01-28T23:11:20.028340+01:00 kenshiro sddm-greeter[1655]: Connected to the daemon.
2016-01-28T23:11:20.028474+01:00 kenshiro sddm[1091]: Message received from greeter: Connect
2016-01-28T23:11:20.062294+01:00 kenshiro sddm-greeter[1655]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
2016-01-28T23:11:20.250847+01:00 kenshiro sddm-greeter[1655]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
2016-01-28T23:11:20.404977+01:00 kenshiro sddm-greeter[1655]: file:///usr/share/sddm/themes/breeze/components/InfoPane.qml:52:22: Unable to assign [undefined] to int
2016-01-28T23:11:20.405245+01:00 kenshiro sddm-greeter[1655]: Adding view for "HDMI2" QRect(0,0 1920x1080)
2016-01-28T23:11:20.408967+01:00 kenshiro sddm-greeter[1655]: Message received from daemon: Capabilities
2016-01-28T23:11:20.409139+01:00 kenshiro sddm-greeter[1655]: Message received from daemon: HostName
**2016-01-28T23:15:03.058840+01:00 kenshiro sddm-helper[1653]: [PAM] Ended.**
**2016-01-28T23:15:03.059337+01:00 kenshiro sddm[1091]: Auth: sddm-helper exited successfully**
**2016-01-28T23:15:03.059517+01:00 kenshiro sddm[1091]: Greeter stopped.**
Version: kcm_sddm-5.5.4-47.1.x86_64