Then I guess the only workable solution would be for you to reinstall. 
 
apparently sddm is not involved (same erreur with startx), kde switches well in graphical mode (“get my theme loading screen”)? so it could be due to user side problem
For test purposes, temporarily rename your ~/.config directory to “~/.config.save”.
And is always good to get infos from logs 
journalctl -b0 -p3
            I renamed it, what should I do now?
I just decided to reinstall
restart plasma !
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