Yesterday I installed the recommended updates with the Muon Update Manager. Then I put my notebook into sleep.
Today I started the PC - and surprisingly it was booting instead of just waking up.
In KDE the notification came:
KDE Power Management System could not be initialized. The backend reported the following error: No valid Power Management backend plugins are available. A new installation might solve this problem. Please check your system configuration.
In the application launcher menu the entries for Sleep, Shut Down and Restart were missing! Also the WIFI didn't work.
I reinstalled (or just reinitialized) upower
sudo apt-get install upower
This brought back the icons in the launcher menu. WIFI is working again.
Restarting the PC… and yeah, it working as it should!