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Now with your problem, you state that you did change the SDDM and Splash screen.
To look at your logs, you can open Konsole CtrlAltT and enter journalctl -p3 -xb
If you paste any content here you must use ‘backticks’ to mark your code…
Backticks ``` are like an apostrophe, find them on the <kbd>~</kbd> ~ key).
So initially, I would like to reset the system to the way it was before installing the SDDM and splash screen.
Also, I would like to know if you enabled snapshots. My system has Timeshift set up to take HOURLY snapshots… so often, if something is wrong and I’m in a hurry and can’t be bothered to figure it out, I will just restore a snapshot.
This is a great trick - imagine if you mess with SDDM and can’t log in at all? You can run Timeshift also from a USB session.
One way to possibly ‘refresh’ to remove the black screen would be to switch to another TTY and back again:
CtrlAlt5 then CtrlAlt1