I finally solved the problem:
in settings > the events calender i found the url slug ‘events”
so the url for the events calender is for instance https://www.yoursite.com/events
in appearance > menu at the settings for the menu button that refers to the events calender (in my case “agenda”) is also an url specified, which was a different one. i changed that to https://www.yoursite.com/events, saved it, and problem solved!
