Hi I haven’t been precise enough.
I figured now out after some testing that the problem applies only to weekly scheduled events. All these weekly events are shown 1 day ahead of the planned schedule.
Fortnightly scheduled events are shown properly.
There is still the issue with a Default “Event” shown everytime the calendar gets refreshed - which I managed to remove by adding some code to the MMM-CalendarWeek excluding events with the Name “Event”