Hello,
I'm running Operating System: openSUSE Leap 15.2
KDE Plasma Version: 5.18.6
KDE Frameworks Version: 5.71.0
Qt Version: 5.12.7
My taskbar is vertically arrange on the right side of the screen.
I would like the date in my system tray in the format dd MMM yy, side-by-side.
However, after startup of KDE the three components of the date appear vertically above each other.
By playing with the width of the taskbar I get back to the desired status which is a bit tedious.
Is there a way of having the appearance fixed so it comes up as desired after each restart?

Try saving the session on logout (in System Settings -> Workspace -> Startup and Shutdown -> Deskop Session)

Sorry, that was exactly my setting, namely to save the previous session. Perhaps my frameworks are a bit outdated. Thanks anyway for your advice!