Comment # 2 on bug 968877 from Matthias Klostermair

I can confirm this very same issue, and steps to reproduce in my almost fresh
KVM VM.

I also tryed cleaning KDE caches for the mainuser initially. - Did not work 
Very fresh user for testing purpose. - Same issue.

Directly after installing the updates the shutdown dialogue already stopped
working.

If you get stuck on the Plasma splashscreen, and need any form of Desktop in
the meantime you can switch to VT e.g.:
CRTL + ALT + F1 and try to get to sddm. e.g. with:
rcxdm stop
rcxdm start

>From there you can select another Desktop session and logon with existing user
account.

Greetings,