Thank you @karsten13 and thank you for the docker image, appreciated.
It seems to be an issue with MMM-CalendarEx2…
It was/is an issue with having multiple MMM-CalendarEx2 modules registered.
I have one page with a week view and another page with a month view (using MMM-Page-Selector).
But since I’m a total MM noob, I just added the CalEx2 module twice and it is apparently not happy about that.
I’ll see if there is a way to have different scenes on different pages or switch to another calendar module for the month view.