Calendar is loading forever

we are using NC 28.0.1 with Calendar 4.6.1
We use 3 different calendars beside the personal one. I don’t know since when, but when I open the calendar app in monthly view now the first 2 calendars are loaded and then the loading of the other calendars stops. I see the buttons of the not loaded calendars moving around for ever. Firefox gives a notice, that the app is slowing down the system.
Any suggestions ?