I've searched extensively and there are many mentions of this problem but it seems only a few disparate users have had it over the last few months. But the 3 Kubuntu systems I upgraded to 20.10 all have it now, so I'm surprised it's not a critical bug with already a solution.
Description:
- 'switch user' is gone from the K / Leave menu
- 'Switch user' is gone from SDDM lock screen (you can only re-enter your own locked session). Shutdown / reboot is also gone from lock screen.
- Maybe unrelated: when logging out some users (always the same ones), I get a black screen (for others I go back to a normal sddm screen with user selection). This is not related to the kde_settings.conf / MinimumUid issue
Out systems are family-shared, so it's really annoying. If the lock screen is on, the system has to be forcefully rebooted for another user to use it, with all the disk corruption risks involved.
Here's the result on one of the 3 systems. I don't see any smoking gun in it:
$ 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 update: after the upgrade to kubuntu 21.10, it finally worked as expected for a few weeks... until some update thrashed that. Now using the Switch User option leads to a black screen you can't recover from. I can still use Ctrl-Alt-F2 to kill and restart plasma, but it's bad. Will post more when I have time to gather data.
In my experience, the simplest workaround for this (until 21.04 comes out) is Ctrl-Alt-F2 (F3, F4,...), log in in console mode and type startx.
I could not find a definitive fix for what, in my opinion, appears to be a major theme flaw for the default Kubuntu KDE plasma desktop.
This was my approach to a workaround:
Add 'User Switcher' widget to the desktop:
Right click on an empty spot in the desktop and from the menu select 'Add Widget'. Type 'user' in the search bar. Add it to the desktop.
Change the default login screen:
Application Launcher->System Setting->Startup and Shutdown+Login Screen(SDDM)
I choose 'Get New Login Screens' and chose one the had a drop menu for the users. In my case it was a screen called 'Sugar Candy'.
Problems I encountered with this workaround:
Locking the session only allowed the locked user to login. To switch user required current user to log back in then use switch user widget.
If the current user started a new session, it would start up an unusable session on tty2. At one point getting stuck and having to reboot. In most cases was able to leave the new session with 'Ctrl+Alt+F1'.
This was tested on both my Kubuntu 20.04. An a new Kubuntu 20.10 in a VM. At some point in 20.04 a 'Switch User' icon suddenly appeared in the application launcher that wasn't there before. Still not sure what caused that.
I will explore KDE with other distros to determine if this is a Kubuntu problem or a KDE problem.
My Opinion:
This would would be a definite deal breaker for a multi user machine. I have been using Kubuntu for many years now as a sole user and love it. But for multiple users I feel it would be complete trash with the current default installation.
Systemd no longer provides "CanMultiSession" property, so software that was using this now breaks :(
KDE fix this in 5.20.3
See: https://bugs.kde.org/show_bug.cgi?id=427777