Session bug at shutdown - Failed to unmount

in KDE session settings I use “always start w/ new session” on shutdown it’s stuck vor about 2 min. and shows:
Shutdown:Failed to unmount /oldroot

dbus-daemon[1209]: [session uid=1000 pid=1209] Activated service ‘org.kde.KSplash’ failed: Process org.kde.KSplash exited with status 1

when I switch between start w/ saved session & start new session in KDE session settings before shutdown, it works and no error message appears …
so currently I have to switch the session settings before a shutdown or reboot