1. I tried to inactivate the other plugins and it didn’t change anything. So I activated them again.
2. I tried to add the widget from the left “personalize” sidebar instead of using the dashboard>appearance>widget menu, and it worked. So the issue was that adding the calendar from the dashboard didn’t work at first, but for some very strange reason, it works now, as if using the left sidebar (personalize) unblocked something.
I cannot find an explanation why this happened… But it works !
Thank you Andrew for looking into this, this is much apreciated !