Hi Christel and welcome to the forums. 🙂
I see what you mean there. You can navigate between months from the mini calendar, but clicking on a date doesn’t seem to produce the list of events for that day the way you’d expect. Definitely not good.
I see you tried deactivating all other plugins–did you notice any difference when you did that?
How about when you try activating a default WordPress theme (like Twenty Fifteen)? Does clicking on the days still produce nothing when you have a default theme active?
Let’s start there and see what we find.
Cheers!
Geoff