I'm running openSUSE Leap 15.2 with KDE using a setup with 3 monitors.

Every few weeks windows in all virtual desktops are suddenly rearranged (each
window is staying in the correct virtual desktop).

In the message log I can see:

Okt 07 09:35:59 jglap kdeinit5[4810]: Service 
"org.kde.StatusNotifierHost-4860" unregistered
Okt 07 09:35:59 jglap drkonqi[25320]: found lsb_release
Okt 07 09:35:59 jglap drkonqi[25320]: Using /proc to determine executable path
Okt 07 09:35:59 jglap drkonqi[25320]: Executable is: "/usr/bin/plasmashell"
Okt 07 09:35:59 jglap drkonqi[25320]: Executable exists: true
Okt 07 09:35:59 jglap drkonqi[25320]: Enabling drkonqi crash catching
Okt 07 09:35:59 jglap kdeinit5[4810]: Registering ":1.3261/StatusNotifierItem"
to system tray
Okt 07 09:35:59 jglap plasmashell[25319]: Aborting shell load: The activity
manager daemon (kactivitymanagerd) is not running.
Okt 07 09:35:59 jglap plasmashell[25319]: If this Plasma has been installed
into a custom prefix, verify that its D-Bus service>
Okt 07 09:35:59 jglap plasmashell[25319]: org.kde.plasmaquick: Applet preload
policy set to 1
...

Using ps I can see kactivitymanagerd being still active:

gross     4895  4714  0 Sep09 ?        00:02:13 /usr/bin/kactivitymanagerd
start-daemon