我进行了广泛的搜索，有很多人提到了这个问题，但似乎只有少数不同的用户在过去几个月里遇到了这个问题。但我升级到 20.10 的 3 个 Kubuntu 系统现在都遇到了这个问题，所以我很惊讶这不是一个已经得到解决方案的严重错误。
描述：
- “切换用户”已从 K/离开菜单中消失
- SDDM 锁定屏幕上的“切换用户”功能已消失（您只能重新进入自己锁定的会话）。关机/重启功能也已消失。
- 可能不相关：注销某些用户（总是相同的用户）时，我会看到黑屏（对于其他用户，我会返回到带有用户选择的正常 sddm 屏幕）。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。如果锁屏处于打开状态，系统必须强制重启才能让其他用户使用，而且还存在磁盘损坏的风险。
以下是 3 个系统之一的结果。我没有看到任何确凿的证据：
$ 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：升级到 kubuntu 21.10 后，它终于按预期运行了几个星期……直到某个更新破坏了它。现在使用切换用户选项会导致黑屏，无法恢复。我仍然可以使用 Ctrl-Alt-F2 来关闭并重新启动 Plasma，但这很糟糕。当我有时间收集数据时，我会发布更多信息。
答案1
根据我的经验，解决此问题最简单的方法（直到 21.04 发布）是按 Ctrl-Alt-F2（F3、F4……），以控制台模式登录并输入 startx。
答案2
Systemd 不再提供“CanMultiSession”属性，因此使用该属性的软件现在会崩溃 :(
KDE 在 5.20.3 中修复了这个问题
答案3
我无法找到一个明确的解决方案，在我看来，这似乎是默认 Kubuntu KDE 等离子桌面的一个主要主题缺陷。
这是我的解决方法：
将“用户切换器”小部件添加到桌面：
右键单击桌面上的空白处，然后从菜单中选择“添加小部件”。在搜索栏中输入“用户”。将其添加到桌面。
更改默认登录屏幕：
应用程序启动器->系统设置->启动和关闭+登录屏幕（SDDM）
我选择“获取新登录屏幕”，然后选择一个带有用户下拉菜单的屏幕。在我的例子中，它是一个名为“Sugar Candy”的屏幕。
我在使用此解决方法时遇到的问题：
锁定会话仅允许被锁定的用户登录。要切换用户，需要当前用户重新登录，然后使用切换用户小部件。
如果当前用户启动了新会话，它将在 tty2 上启动不可用的会话。有时会卡住并不得不重新启动。在大多数情况下，可以使用“Ctrl+Alt+F1”退出新会话。
这是我在 Kubuntu 20.04 和虚拟机中的新 Kubuntu 20.10 上测试的。在 20.04 的某个时刻，应用程序启动器中突然出现了一个之前没有的“切换用户”图标。仍然不确定是什么原因造成的。
我将与其他发行版一起探索 KDE，以确定这是 Kubuntu 问题还是 KDE 问题。
我的想法：
对于多用户机器来说，这绝对是一个大问题。作为唯一用户，我已经使用 Kubuntu 很多年了，并且非常喜欢它。但对于多用户来说，我觉得当前的默认安装会是彻底的垃圾。