Bug#1021985: sddm login screen changed to white and does not display the breeze theme any more
Here some entries from an sddm start in /var/log/syslog, filtered by grep sddm:
Oct 18 08:46:21 betty sddm[925]: Initializing...
Oct 18 08:46:21 betty sddm[925]: Starting...
Oct 18 08:46:21 betty sddm[925]: Logind interface found
Oct 18 08:46:21 betty sddm[925]: Adding new display on vt 7 ...
Oct 18 08:46:21 betty sddm[925]: Loading theme configuration from ""
Oct 18 08:46:21 betty sddm[925]: Display server starting...
Oct 18 08:46:21 betty sddm[925]: Adding cookie to "/var/run/sddm/{aa4bb32b-dbfe-438e-8bde-4348c400857f}"
Oct 18 08:46:21 betty sddm[925]: Running: /usr/bin/X -dpi 0 -auth /var/run/sddm/{aa4bb32b-dbfe-438e-8bde-4348c400857f} -background none -noreset -displayfd 17 -seat seat0 vt7
Oct 18 08:46:22 betty sddm[925]: Setting default cursor
Oct 18 08:46:22 betty sddm[925]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
Oct 18 08:46:22 betty sddm[925]: Display server started.
Oct 18 08:46:22 betty sddm[925]: Socket server starting...
Oct 18 08:46:22 betty sddm[925]: Socket server started.
Oct 18 08:46:22 betty sddm[925]: Loading theme configuration from "/usr/share/sddm/themes/breeze/theme.conf"
Oct 18 08:46:22 betty sddm[925]: Greeter starting...
Oct 18 08:46:22 betty sddm-helper[1386]: [PAM] Starting...
Oct 18 08:46:22 betty sddm-helper[1386]: [PAM] Authenticating...
Oct 18 08:46:22 betty sddm-helper[1386]: [PAM] returning.
Oct 18 08:46:23 betty systemd[1388]: Cleanup lingering KCrash metadata was skipped because of a failed condition check (ConditionPathExistsGlob=/var/lib/sddm/.cache/kcrash-metadata/*.ini).
Oct 18 08:46:23 betty systemd[1388]: Cleanup lingering KCrash metadata was skipped because of a failed condition check (ConditionPathExistsGlob=/var/lib/sddm/.cache/kcrash-metadata/*.ini).
Oct 18 08:46:23 betty systemd[1]: Started Session 1 of User sddm.
Oct 18 08:46:23 betty sddm[925]: Greeter session started successfully
Oct 18 08:46:23 betty sddm-greeter[1408]: High-DPI autoscaling not Enabled
Oct 18 08:46:23 betty sddm-greeter[1408]: Reading from "/usr/share/xsessions/plasma.desktop"
Oct 18 08:46:23 betty sddm-greeter[1408]: Loading theme configuration from "/usr/share/sddm/themes/breeze/theme.conf"
Oct 18 08:46:23 betty sddm-greeter[1408]: Connected to the daemon.
Oct 18 08:46:23 betty sddm[925]: Message received from greeter: Connect
Oct 18 08:46:23 betty sddm-greeter[1408]: QFont::fromString: Invalid description '(empty)'
Oct 18 08:46:23 betty sddm-greeter[1408]: Loading file:///usr/share/sddm/themes/breeze/Main.qml...
Oct 18 08:46:23 betty sddm-greeter[1408]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is QGuiApplication(0x7fffced42270), parent's thread is QThread(0x55c623077060), current thread is QThread(0x55c62319ad10)
Oct 18 08:46:23 betty sddm-greeter[1408]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is QGuiApplication(0x7fffced42270), parent's thread is QThread(0x55c623077060), current thread is QThread(0x55c62319ad10)
Oct 18 08:46:23 betty sddm-greeter[1408]: QObject: Cannot create children for a parent that is in a different thread.#012(Parent is QGuiApplication(0x7fffced42270), parent's thread is QThread(0x55c623077060), current thread is QThread(0x55c62319ad10)
Oct 18 08:46:23 betty sddm-greeter[1408]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Oct 18 08:46:23 betty dbus-daemon[674]: [system] Activating via systemd: service name='org.freedesktop.UPower' unit='upower.service' requested by ':1.34' (uid=115 pid=1408 comm="/usr/bin/sddm-greeter --socket /tmp/sddm-:0-PRpPZo")
Oct 18 08:46:23 betty sddm-greeter[1408]: Failed to find a Kirigami platform plugin
Oct 18 08:46:23 betty sddm-greeter[1408]: file:///usr/share/sddm/themes/breeze/components/VirtualKeyboard.qml:12:1: Type InputPanel unavailable
Oct 18 08:46:23 betty sddm-greeter[1408]: qrc:/QtQuick/VirtualKeyboard/content/InputPanel.qml:127:5: Type Keyboard unavailable
Oct 18 08:46:23 betty sddm-greeter[1408]: qrc:/QtQuick/VirtualKeyboard/content/components/Keyboard.qml:38:1: module "QtQuick.VirtualKeyboard.Plugins" is not installed
Oct 18 08:46:23 betty sddm-greeter[1408]: file:///usr/share/sddm/themes/breeze/Background.qml:21:5: QML Image: Cannot open: file:///usr/share/wallpapers/Next/contents/images/5120x2880.jpg
Oct 18 08:46:23 betty sddm-greeter[1408]: Adding view for "HDMI-1" QRect(0,0 1600x1200)
Oct 18 08:46:23 betty sddm-greeter[1408]: Qt Quick Layouts: Polish loop detected. Aborting after two iterations.
Oct 18 08:46:24 betty sddm-greeter[1408]: Message received from daemon: Capabilities
Oct 18 08:46:24 betty sddm-greeter[1408]: Message received from daemon: HostName
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