Finally the ability of the clock/calendar widget to display your PIM events came back in the latest update of KDE/Plasma :)
You need at least the following versions:
- „KDE Plasma“ >=5.7.4
- „KDE Frameworks“ >=5.25.0
You can check this when you start „kinfocenter“.
To be able to see your events you must enable the PIM events plugin for your widget.