Do you have the event calendar plasmoid running?
If so please update it because the old one did not work on new qt-5.15.....
https://github.com/Zren/plasma-applet-eventcalendarIf not, rename in ~/.config the "plasma-org.kde.plasma.desktop-appletsrc" and try to login again.
If it works you can diff it.
So I found out, the event calendar plasmoid was the reason why........
EDIT
Oh, sorry i didn't read all of your post.
Forget the words i wrote above!
It didn't reach the login manger (sddm?) after booting?
And yes it could be the reason, what does a full-upgrade/d-u will remove?
And please give us the output of inxi -v1a ( please use the [code-tags] output [/code-tag] without the "-tag".