That didn’t help. But I deleted the calendar and then inserted it again.
Now it works! At least, I can select by day, event, and change the month.
But there is buggy rendering if I change the “Available views” to Month only:
The calendar fills the page width, the month left arrow and today button appear twice. My “+” icon has moved to the left of the page (in the Design it is on the right). Maybe the calendar element can’t handle a small width?
If I change back to Day-month-week view option then it looks a bit better but is still not right: