I have the following issue with a fresh arch-linux installation: in KDE/plasma with SDDM neither su or sudo work. I tried to use them in both konsole and xterm. What does also not work is authentication after the screen locker starts (this means I cannot unlock the screen). Initial login with SDDM works. (So to "unlock" the session I have to restart the SDDM service).
If I switch to another session (using CTRL-F2, etc.) I can use su and sudo as expected there.
What I have tried so far is reinstalling 'pambase' and 'pam' and check the configuration for /etc/pam.d/system-auth:
#%PAM-1.0 auth required pam_unix.so try_first_pass nullok auth optional pam_permit.so auth required pam_env.so account required pam_unix.so account optional pam_permit.so account required pam_time.so password required pam_unix.so try_first_pass nullok sha512 shadow password optional pam_permit.so session required pam_limits.so session required pam_unix.so session optional pam_permit.so
And of course also /etc/pam.d/sddm:
#%PAM-1.0 auth include system-login account include system-login password include system-login session include system-login
What might be interessting is the output of journalctl after calling su:
Apr 28 14:42:30 zenbook su: pam_unix(su:auth): authentication failure; logname=myuser uid=1000 euid=0 tty=pts/2 ruser=myuser rhost= user= Apr 28 14:42:32 zenbook su: FAILED SU (to root) myuser on pts/2
Or after calling 'sudo nano':
Apr 28 15:06:29 zenbook sudo: pam_unix(sudo:auth): authentication failure; logname=myuser uid=1000 euid=0 tty=/dev/pts/2 rfuser=myuser rhos Apr 28 15:06:43 zenbook sudo: myuser : 3 incorrect password attempts ; TTY=pts/2 ; PWD=/home/myuser ; USER=root ; COMMAND=/usr/bin/nano
I don't know what further information would be helpful to resolve this issue. I would be happy to provide more information if required.
Note: I have also asked the question on the arch forum without getting a reply so far.
So far I think it is related to SDDM because I have the same issue for example in Enlitenment when run with SDDM. If I start however an X11 session without SDDM su and sudo work without problems.