我进行了广泛的搜索，有很多关于这个问题的提及，但在过去几个月中只有少数几个不同的用户遇到过。但是我将3台Kubuntu系统升级到20.10后，现在都有这个问题，所以我很惊讶它不是一个已经有解决方案的关键性错误。
描述：
- “切换用户”选项从K/离开菜单中消失了
- 登录界面的SDDM锁屏也没有“切换用户”选项（您只能重新进入自己的已锁定会话）。关机/重启选项也从锁屏界面中消失了。
- 可能与此无关：注销某些用户时（始终是同一些用户），我得到一个黑屏（对于其他用户，我返回到正常的sddm屏幕并可以选择用户）。这与kde_settings.conf / MinimumUid问题无关
我们的系统是家庭共享的，所以这真的很烦人。如果锁屏界面打开，必须强制重新启动系统才能让其他用户使用它，这涉及到所有与磁盘损坏风险相关的问题。
这是其中一个3台系统的结果。我没有看到任何明显的问题：
$ journalctl --since "2020-12-25" -u sddm | sed -e "s/.* systemname //"
-- Logs begin at Tue 2019-12-10 19:42:43 CET, end at Sun 2021-01-10 12:09:49 CET. --
systemd[1]: Starting Simple Desktop Display Manager...
systemd[1]: Started Simple Desktop Display Manager.
sddm[2035]: Initializing...
sddm[2035]: Starting...
sddm[2035]: Logind interface found
sddm[2035]: Adding new display on vt 1 ...
sddm[2035]: Loading theme configuration from ""
sddm[2035]: Display server starting...
sddm[2035]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{2eab554e-43ae-4c2b-8751-b3bea3a04ef0} -background none -noreset -displayfd 17 -seat seat0 vt1
sddm[2035]: Setting default cursor
sddm[2035]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
sddm[2035]: Display server started.
sddm[2035]: Socket server starting...
sddm[2035]: Socket server started.
sddm[2035]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
sddm[2035]: Greeter starting...
sddm[2035]: Adding cookie to "/var/run/sddm/{2eab554e-43ae-4c2b-8751-b3bea3a04ef0}"
sddm-helper[2353]: [PAM] Starting...
sddm-helper[2353]: [PAM] Authenticating...
sddm-helper[2353]: [PAM] returning.
sddm-helper[2353]: pam_unix(sddm-greeter:session): session opened for user sddm by (uid=0)
sddm[2035]: Greeter session started successfully
sddm[2035]: Message received from greeter: Connect
sddm[2035]: Message received from greeter: Login
sddm[2035]: Reading from "/usr/share/xsessions/plasma.desktop"
sddm[2035]: Reading from "/usr/share/xsessions/plasma.desktop"
sddm[2035]: Session "plasma.desktop" selected, command: "/usr/bin/startplasma-x11"
sddm-helper[2424]: [PAM] Starting...
sddm-helper[2424]: [PAM] Authenticating...
sddm-helper[2424]: [PAM] Preparing to converse...
sddm-helper[2424]: [PAM] Conversation with 1 messages
sddm-helper[2424]: gkr-pam: unable to locate daemon control file
sddm-helper[2424]: gkr-pam: stashed password to try later in open session
sddm-helper[2424]: [PAM] returning.
sddm-helper[2424]: pam_kwallet5(sddm:auth): (null): pam_sm_authenticate
sddm[2035]: Authenticated successfully
sddm-helper[2424]: pam_kwallet5(sddm:setcred): pam_kwallet5: pam_sm_setcred
sddm-helper[2424]: pam_unix(sddm:session): session opened for user dargaud by (uid=0)
sddm[2035]: Auth: sddm-helper exited successfully
sddm[2035]: Greeter stopped.
sddm-helper[2424]: gkr-pam: gnome-keyring-daemon started properly and unlocked keyring
sddm-helper[2424]: pam_kwallet5(sddm:session): pam_kwallet5: pam_sm_open_session
sddm-helper[2424]: Starting: "/etc/sddm/Xsession \"/usr/bin/startplasma-x11\""
sddm[2035]: Session started
sddm[2035]: Authentication error: "Process crashed"
sddm[2035]: Auth: sddm-helper crashed (exit code 15)
sddm[2035]: Authentication error: "Process crashed"
sddm[2035]: Auth: sddm-helper exited with 15
sddm[2035]: Socket server stopping...
sddm[2035]: Socket server stopped.
sddm[2035]: Display server stopping...
systemd[1]: Stopping Simple Desktop Display Manager...
sddm[2035]: Display server stopped.
sddm[2035]: Running display stop script "/usr/share/sddm/scripts/Xstop"
sddm[2035]: Removing display ":0" ...
sddm[2035]: Adding new display on vt 1 ...
sddm[2035]: Loading theme configuration from ""
sddm[2035]: Display server starting...
sddm[2035]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{5d343de6-e3c5-4c04-8659-a45f6125ce5f} -background none -noreset -displayfd 18 -seat seat0 vt1
sddm[2035]: Setting default cursor
sddm[2035]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
sddm[2035]: Display server started.
sddm[2035]: Socket server starting...
sddm[2035]: Socket server started.
sddm[2035]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
sddm[2035]: Greeter starting...
sddm[2035]: Adding cookie to "/var/run/sddm/{5d343de6-e3c5-4c04-8659-a45f6125ce5f}"
sddm[2035]: Signal received: SIGTERM
sddm[2035]: Socket server stopping...
sddm[2035]: Socket server stopped.
sddm[2035]: Display server stopping...
sddm-helper[9047]: [PAM] Starting...
sddm-helper[9047]: [PAM] Authenticating...
sddm-helper[9047]: [PAM] returning.
sddm[2035]: Display server stopped.
sddm[2035]: Running display stop script "/usr/share/sddm/scripts/Xstop"
sddm[2035]: QProcess: Destroyed while process ("/usr/lib/x86_64-linux-gnu/sddm/sddm-helper") is still running.
systemd[1]: sddm.service: Succeeded.
systemd[1]: Stopped Simple Desktop Display Manager.
-- Reboot --
systemd[1]: Starting Simple Desktop Display Manager...
systemd[1]: Started Simple Desktop Display Manager.
sddm[1934]: Initializing...
sddm[1934]: Starting...
sddm[1934]: Logind interface found
sddm[1934]: Adding new display on vt 1 ...
sddm[1934]: Loading theme configuration from ""
sddm[1934]: Display server starting...
sddm[1934]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{d57df6b7-2776-4dc6-b28b-1746e9a6613d} -background none -noreset -displayfd 17 -seat seat0 vt1
sddm[1934]: Setting default cursor
sddm[1934]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
sddm[1934]: Display server started.
sddm[1934]: Socket server starting...
sddm[1934]: Socket server started.
sddm[1934]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
sddm[1934]: Greeter starting...
sddm[1934]: Adding cookie to "/var/run/sddm/{d57df6b7-2776-4dc6-b28b-1746e9a6613d}"
sddm-helper[2206]: [PAM] Starting...
sddm-helper[2206]: [PAM] Authenticating...
sddm-helper[2206]: [PAM] returning.
sddm-helper[2206]: pam_unix(sddm-greeter:session): session opened for user sddm by (uid=0)
sddm[1934]: Greeter session started successfully
sddm[1934]: Message received from greeter: Connect
sddm[1934]: Message received from greeter: Login
sddm[1934]: Reading from "/usr/share/xsessions/plasma.desktop"
sddm[1934]: Reading from "/usr/share/xsessions/plasma.desktop"
sddm[1934]: Session "plasma.desktop" selected, command: "/usr/bin/startplasma-x11"
sddm-helper[2276]: [PAM] Starting...
sddm-helper[2276]: [PAM] Authenticating...
sddm-helper[2276]: [PAM] Preparing to converse...
sddm-helper[2276]: [PAM] Conversation with 1 messages
sddm-helper[2276]: gkr-pam: unable to locate daemon control file
sddm-helper[2276]: gkr-pam: stashed password to try later in open session
sddm-helper[2276]: [PAM] returning.
sddm-helper[2276]: pam_kwallet5(sddm:auth): (null): pam_sm_authenticate
sddm[1934]: Authenticated successfully
sddm-helper[2276]: pam_kwallet5(sddm:setcred): pam_kwallet5: pam_sm_setcred
sddm-helper[2276]: pam_unix(sddm:session): session opened for user dargaud by (uid=0)
sddm[1934]: Auth: sddm-helper exited successfully
sddm[1934]: Greeter stopped.
sddm-helper[2276]: gkr-pam: gnome-keyring-daemon started properly and unlocked keyring
sddm-helper[2276]: pam_kwallet5(sddm:session): pam_kwallet5: pam_sm_open_session
sddm-helper[2276]: Starting: "/etc/sddm/Xsession \"/usr/bin/startplasma-x11\""
sddm[1934]: Session started
systemd[1]: Stopping Simple Desktop Display Manager...
sddm[1934]: Signal received: SIGTERM
sddm[1934]: Authentication error: "Process crashed"
sddm[1934]: Auth: sddm-helper crashed (exit code 15)
sddm[1934]: Authentication error: "Process crashed"
sddm[1934]: Auth: sddm-helper exited with 15
sddm[1934]: Socket server stopping...
sddm[1934]: Socket server stopped.
sddm[1934]: Display server stopping...
sddm[1934]: Display server stopped.
sddm[1934]: Running display stop script "/usr/share/sddm/scripts/Xstop"
sddm[1934]: Removing display ":0" ...
sddm[1934]: Adding new display on vt 1 ...
sddm[1934]: Loading theme configuration from ""
sddm[1934]: Display server starting...
sddm[1934]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{784e9002-ceaa-453f-9bbe-8c596e312df0} -background none -noreset -displayfd 18 -seat seat0 vt1
sddm[1934]: Failed to read display number from pipe
sddm[1934]: Display server failed to start. Exiting
systemd[1]: sddm.service: Main process exited, code=killed, status=6/ABRT
systemd[1]: sddm.service: Failed with result 'signal'.
systemd[1]: Stopped Simple Desktop Display Manager.
systemd[1]: sddm.service: Triggering OnFailure= dependencies.
systemd[1]: sddm.service: Failed to enqueue OnFailure= job, ignoring: Transaction for plymouth-quit.service/start is destructive (systemd-tmpfiles-setup-dev.service has 'stop' job queued, but 'start' is included in transaction).
-- Reboot --
systemd[1]: Starting Simple Desktop Display Manager...
systemd[1]: Started Simple Desktop Display Manager.
sddm[2066]: Initializing...
sddm[2066]: Starting...
sddm[2066]: Logind interface found
sddm[2066]: Adding new display on vt 1 ...
sddm[2066]: Loading theme configuration from ""
sddm[2066]: Display server starting...
sddm[2066]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{f4e5de63-9bec-4555-b1c8-b622830fff8f} -background none -noreset -displayfd 17 -seat seat0 vt1
sddm[2066]: Setting default cursor
sddm[2066]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
sddm[2066]: Display server started.
sddm[2066]: Socket server starting...
sddm[2066]: Socket server started.
sddm[2066]: Loading theme configuration from "/usr/share/sddm/themes/ubuntu-theme/theme.conf"
sddm[2066]: Greeter starting...
sddm[2066]: Adding cookie to "/var/run/sddm/{f4e5de63-9bec-4555-b1c8-b622830fff8f}"
sddm-helper[2344]: [PAM] Starting...
sddm-helper[2344]: [PAM] Authenticating...
sddm-helper[2344]: [PAM] returning.
sddm-helper[2344]: pam_unix(sddm-greeter:session): session opened for user sddm by (uid=0)
sddm[2066]: Greeter session started successfully
sddm[2066]: Message received from greeter: Connect
sddm[2066]: Message received from greeter: Login
sddm[2066]: Reading from "/usr/share/xsessions/plasma.desktop"
sddm[2066]: Reading from "/usr/share/xsessions/plasma.desktop"
sddm[2066]: Session "plasma.desktop" selected, command: "/usr/bin/startplasma-x11"
sddm-helper[2414]: [PAM] Starting...
sddm-helper[2414]: [PAM] Authenticating...
sddm-helper[2414]: [PAM] Preparing to converse...
sddm-helper[2414]: [PAM] Conversation with 1 messages
sddm-helper[2414]: gkr-pam: unable to locate daemon control file
sddm-helper[2414]: gkr-pam: stashed password to try later in open session
sddm-helper[2414]: [PAM] returning.
sddm-helper[2414]: pam_kwallet5(sddm:auth): (null): pam_sm_authenticate
sddm[2066]: Authenticated successfully
sddm-helper[2414]: pam_kwallet5(sddm:setcred): pam_kwallet5: pam_sm_setcred
sddm-helper[2414]: pam_unix(sddm:session): session opened for user dargaud by (uid=0)
sddm[2066]: Auth: sddm-helper exited successfully
sddm[2066]: Greeter stopped.
sddm-helper[2414]: gkr-pam: gnome-keyring-daemon started properly and unlocked keyring
sddm-helper[2414]: pam_kwallet5(sddm:session): pam_kwallet5: pam_sm_open_session
sddm-helper[2414]: Starting: "/etc/sddm/Xsession \"/usr/bin/startplasma-x11\""
sddm[2066]: Session started
2021年更新：在升级到Kubuntu 21.10之后，它终于按预期工作了几周...直到某个更新破坏了它。现在使用“切换用户”选项会导致一个无法恢复的黑屏。我仍然可以使用Ctrl-Alt-F2来终止和重新启动Plasma，但这很糟糕。等我有时间收集数据时会发布更多信息。
journalctl -u sddm- da_kingpinjournalctl --since "2020-12-25" -u sddm。这样可以查看自圣诞节以来的日志。可能与'systemd-logind'服务有关。 - da_kingpin