Hm, thats weird. I didnt change anything to SDDM.
Journal seems to be OK too
Summary
sddm[884]: Initializing…
sddm[884]: Starting…
sddm[884]: Logind interface found
sddm[884]: Adding new display…
sddm[884]: Loaded empty theme configuration
sddm[884]: Xauthority path: “/run/sddm/xauth_ZdKZqn”
sddm[884]: Using VT 2
sddm[884]: Display server starting…
sddm[884]: Writing cookie to “/run/sddm/xauth_ZdKZqn”
sddm[884]: Running: /usr/bin/X -dpi 0 -background none -seat seat0 vt2 -auth /run/sddm/xauth_ZdKZqn -noreset -displayfd 16
sddm[884]: Setting default cursor
sddm[884]: Could not setup default cursor
sddm[884]: Running display setup script “/usr/share/sddm/scripts/Xsetup”
sddm[884]: Display server started.
sddm[884]: Socket server starting…
sddm[884]: Socket server started.
sddm[884]: Loading theme configuration from “/usr/share/sddm/themes/breath/theme.conf”
sddm[884]: Greeter starting…
sddm-helper[893]: [PAM] Starting…
sddm-helper[893]: [PAM] Authenticating…
sddm-helper[893]: [PAM] returning.
After loggin in via tty and start plasma, lock the screen with SUPER+L
it seems to work again, even after reboot.
Now my second monitor is gone.
There is a Popup for the second monitor to be plugged in or removed, but i cant add it as an output device anymore.
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See How to Post Command Output as Preformatted Text 