系统正常启动，然后我登录。我打开 xterm，会发生以下情况：
$ groups
rphelps
$ su rphelps
Password:
$ groups
rphelps adm cdrom sudo dip www-data plugdev lpadmin sambashare kismet docker
如果我通过 SSH 登录，我的组设置正确。如果我登录到 tty 而不是 X，则我的组设置正确。
当我尝试注销 X 会话或按 Ctrl+Alt+L 时，它会关闭会话，但似乎问候语无法（重新）启动，或者锁屏不起作用，并且它让我回到桌面。
我最近正在摆弄屏幕保护程序（xscreensaver、gnome-screensaver 等），试图让屏幕在 30 分钟后锁定（没有运气）。清除这些内容是否删除了对于注销至关重要的内容？
我没有/etc/手动修改任何文件。
Xubuntu 18.04
编辑1：
$ journalctl -u lightdm.service
-- Reboot --
Jul 25 11:21:01 rphelps-DELL systemd[1]: Starting Light Display Manager...
Jul 25 11:21:01 rphelps-DELL lightdm[993]: Seat type 'xlocal' is deprecated, use 'type=local' instead
Jul 25 11:21:01 rphelps-DELL systemd[1]: Started Light Display Manager.
Jul 25 11:21:02 rphelps-DELL lightdm[1179]: pam_kwallet(lightdm-greeter:setcred): (null): pam_sm_setcred
Jul 25 11:21:02 rphelps-DELL lightdm[1179]: pam_kwallet5(lightdm-greeter:setcred): (null): pam_sm_setcred
Jul 25 11:21:02 rphelps-DELL lightdm[1179]: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)
Jul 25 11:21:03 rphelps-DELL lightdm[1179]: pam_kwallet(lightdm-greeter:session): (null): pam_sm_open_session
Jul 25 11:21:03 rphelps-DELL lightdm[1179]: pam_kwallet(lightdm-greeter:session): pam_kwallet: open_session called without kwallet_key
Jul 25 11:21:03 rphelps-DELL lightdm[1179]: pam_kwallet5(lightdm-greeter:session): (null): pam_sm_open_session
Jul 25 11:21:03 rphelps-DELL lightdm[1179]: pam_kwallet5(lightdm-greeter:session): pam_kwallet5: open_session called without kwallet5_key
Jul 25 11:21:03 rphelps-DELL lightdm[1854]: pam_succeed_if(lightdm:auth): incomplete condition detected
Jul 25 11:22:37 rphelps-DELL lightdm[1854]: pam_kwallet(lightdm:auth): (null): pam_sm_authenticate
Jul 25 11:22:37 rphelps-DELL lightdm[1854]: pam_kwallet5(lightdm:auth): (null): pam_sm_authenticate
Jul 25 11:22:37 rphelps-DELL lightdm[1854]: pam_kwallet(lightdm:setcred): pam_kwallet: pam_sm_setcred
Jul 25 11:22:37 rphelps-DELL lightdm[1854]: pam_kwallet5(lightdm:setcred): pam_kwallet5: pam_sm_setcred
Jul 25 11:22:37 rphelps-DELL lightdm[1854]: pam_unix(lightdm:session): session opened for user rphelps by (uid=0)
Jul 25 11:22:37 rphelps-DELL lightdm[1854]: pam_kwallet(lightdm:session): pam_kwallet: pam_sm_open_session
Jul 25 11:22:37 rphelps-DELL lightdm[993]: Error writing to daemon: Bad file descriptor
Jul 25 11:22:37 rphelps-DELL lightdm[993]: Failed to write utmpx: Permission denied
Jul 25 11:34:02 rphelps-DELL lightdm[993]: Failed to write utmpx: Permission denied
Jul 25 11:34:02 rphelps-DELL lightdm[993]: Error writing to daemon: Bad file descriptor
Jul 25 11:34:02 rphelps-DELL lightdm[993]: Failed to write utmpx: Permission denied
Jul 25 11:40:38 rphelps-DELL lightdm[993]: Failed to write utmpx: Permission denied
Jul 25 11:40:38 rphelps-DELL lightdm[993]: Error writing to daemon: Bad file descriptor
Jul 25 11:40:38 rphelps-DELL lightdm[993]: Failed to write utmpx: Permission denied
Jul 25 11:50:05 rphelps-DELL lightdm[993]: Failed to write utmpx: Permission denied
Jul 25 11:50:05 rphelps-DELL systemd[1]: Stopping Light Display Manager...
Jul 25 11:50:05 rphelps-DELL lightdm[993]: Error updating user /org/freedesktop/Accounts/User1000: GDBus.Error:org.freedesktop.systemd1.ShuttingDown: Refusing
Jul 25 11:50:05 rphelps-DELL lightdm[993]: seat_lock: assertion 'seat != NULL' failed
Jul 25 11:50:05 rphelps-DELL lightdm[993]: Error updating user /org/freedesktop/Accounts/User1000: GDBus.Error:org.freedesktop.systemd1.ShuttingDown: Refusing
Jul 25 11:50:05 rphelps-DELL lightdm[993]: Error updating user /org/freedesktop/Accounts/User1000: GDBus.Error:org.freedesktop.systemd1.ShuttingDown: Refusing
Jul 25 11:50:07 rphelps-DELL systemd[1]: Stopped Light Display Manager.