KDE, everything went fine.
I first had a “file exists” error on some files in '/usr/lib/qt/qml/QtQuick/Controls/' and its subfolders.
→ I removed them.
I had some pacnews:
However, by using them, I couldn't login anymore (black screen, not even Plymouth started).
I reverted them back*, and everything's fine!
So if you have no splashscreen and the login screen never comes, use a Live USB to boot and revert back* to the not-pacnew files.
*: Changing tty doesn't help, logging in is impossible due to the pacnew files not working.