Ubuntu 19.04; 起動時に自動グラフィカルログイン画面が表示されない

Ubuntu 19.04; 起動時に自動グラフィカルログイン画面が表示されない

最近、Ubuntu 19.04 を新規インストールしました。それ以来、マシンを起動すると、ブート ログを示す画面が表示されますが、ログイン画面に切り替わりません。

[CTRL][ALT][F2] を押してから [CTRL][ALT][F1] を押すと、手動でログイン画面を起動できます。

ログイン画面を自動的に表示するにはどうすればよいですか?

ブート ログ (/var/log/boot.log から取得されますが、ブート中に表示される内容と一致します) は次のとおりです。

[  OK  ] Started Tell Plymouth To Write Out Runtime Data.
[  OK  ] Reached target System Initialization.
[  OK  ] Started Discard unused blocks once a week.
[  OK  ] Started CUPS Scheduler.
[  OK  ] Started Trigger anacron every hour.
[  OK  ] Listening on Activation socket for spice guest agent daemon.
         Starting Socket activation for snappy daemon.
[  OK  ] Listening on ACPID Listen Socket.
[  OK  ] Started Daily rotation of log files.
[  OK  ] Started Daily man-db regeneration.
[  OK  ] Listening on UUID daemon activation socket.
[  OK  ] Started ACPI Events Check.
[  OK  ] Reached target Paths.
[  OK  ] Started Message of the Day.
[  OK  ] Listening on D-Bus System Message Bus Socket.
[  OK  ] Listening on Avahi mDNS/DNS-SD Stack Activation Socket.
[  OK  ] Started Daily Cleanup of Temporary Directories.
[  OK  ] Listening on CUPS Scheduler.
[  OK  ] Started Daily apt download activities.
[  OK  ] Started Daily apt upgrade and clean activities.
[  OK  ] Reached target Timers.
[  OK  ] Listening on Socket activation for snappy daemon.
[  OK  ] Reached target Sockets.
[  OK  ] Reached target Basic System.
         Starting Accounts Service...
         Starting LSB: Record successful boot for GRUB...
         Starting Avahi mDNS/DNS-SD Stack...
[  OK  ] Started ACPI event daemon.
         Starting Disk Manager...
[  OK  ] Started irqbalance daemon.
         Starting Switcheroo Control Proxy service...
         Starting Bluetooth service...
         Starting Rotate log files...
         Starting Login Service...
[  OK  ] Started Run anacron jobs.
         Starting LSB: automatic crash report generation...
         Starting Dispatcher daemon for systemd-networkd...
[  OK  ] Reached target Login Prompts.
[  OK  ] Started Save initial kernel messages after boot.
         Starting Detect the available GPUs and deal with any system changes...
[  OK  ] Reached target Sound Card.
         Starting System Logging Service...
         Starting Modem Manager...
         Starting Daily man-db regeneration...
[  OK  ] Started D-Bus System Message Bus.
         Starting Network Manager...
         Starting WPA supplicant...
[  OK  ] Started Regular background program processing daemon.
         Starting Configure Plugged-In Printer...
         Starting Thermal Daemon Service...
[  OK  ] Started CUPS Scheduler.
[  OK  ] Reached target Printer.
         Starting Restore /etc/resolv.conf if the system crashed before the ppp link was shut down...
[  OK  ] Started Set the CPU Frequency Scaling governor.
         Starting Snappy daemon...
[  OK  ] Started Switcheroo Control Proxy service.
[  OK  ] Started Restore /etc/resolv.conf if the system crashed before the ppp link was shut down.
[  OK  ] Started Login Service.
[  OK  ] Started LSB: Record successful boot for GRUB.
[  OK  ] Started LSB: automatic crash report generation.
[  OK  ] Started Detect the available GPUs and deal with any system changes.
[  OK  ] Started Bluetooth service.
[  OK  ] Started WPA supplicant.
[  OK  ] Started Thermal Daemon Service.
[  OK  ] Started System Logging Service.
[  OK  ] Started Avahi mDNS/DNS-SD Stack.
[  OK  ] Started Make remote CUPS printers available locally.
         Starting Manage, Install and Generate Color Profiles...
[  OK  ] Reached target Bluetooth.
         Starting Authorization Manager...
[  OK  ] Started Dispatcher daemon for systemd-networkd.
         Starting Hostname Service...
[  OK  ] Started Authorization Manager.
[  OK  ] Started Hostname Service.
[  OK  ] Started Manage, Install and Generate Color Profiles.
[  OK  ] Started Network Manager.
         Starting Network Manager Wait Online...
[  OK  ] Reached target Network.
         Starting Permit User Sessions...
         Starting OpenVPN service...
[  OK  ] Started Unattended Upgrades Shutdown.
[  OK  ] Started OpenVPN service.
         Stopping CUPS Scheduler...
[  OK  ] Stopped CUPS Scheduler.
[  OK  ] Started CUPS Scheduler.
[  OK  ] Started Permit User Sessions.
         Starting GNOME Display Manager...
         Starting Hold until boot process finishes up...
         Starting Network Manager Script Dispatcher Service...
[  OK  ] Started Network Manager Script Dispatcher Service.
[  OK  ] Started Modem Manager.
[  OK  ] Started GNOME Display Manager.

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