The calendar ist not available

I cannot connect to the calendar with Thunderbird.

I made an import from an *.ics file to nextclooud calendar, that worked fine. I can see everything int the browser.

Then I got to the Thunderbird lightning calendar and set up a new remote calendar with CalDav filling in the path showed under the settings in nextcloud calendar (the primary one). I confirmed the message about the selfsigned certificate and then I entered the username/password of my nextcloud account in the message popped up in Thunderbird. The new calendar ist there but it tells me that is is not available.

Did I miss something essential?