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I fixed this by switching back to gnome, and selecting gdm3 instead of sddm as the Display Manager. I think this was all because I was migrating...
Running `startx` from the console (Ctrl + Alt + F2) works, and KDE boots. I'm guessing this is all because I was setup to use KDE w/ 18.04 LTS...
I've got the same problem here. The problem is switching to GUI mode / X-Windows greeter. I'm not sure what change there was coming from 18.04...