I had a similar issue after a recent update. Just as a test, I
created a
new user (i.e. fresh home directory) and that worked fine, so I figured
it might be something in my existing profile that was an issue. I
cleaned up a bunch of temp and cache files that looked KDE/Plasma
related and the next time I logged in, everything was fine. I wish I
remembered exactly what I did...
That's not my case, I've now reinstalled the whole system from scratch and
I'm still having the issue :-( same problem if I try to create a new user and login
with it.