WebApp "upcoming appointments" widget is empty

Hello community,
The widget upcoming appointments is empty. No future appointments will be displayed even though the calendar is full.
Any idea?

@davodego Could you give more information, such as version, steps to reproduce etc.

WebApp:
3.5.1.2033+1122.1
Kopano Core:
8.7.80

Add widget “upcoming appointments” on today view. Widget ist empty.

The widget shows items from your own default calendar.
Are the appointments in the default calendar?