Hello there,
there is an issue with the NextCloud 11.0.2 Calendar, which obviously not returning "calendar-access" in the header.
I've added return true; // hotfix after $valid_caldav_server = isset($dav_options['calendar-access']); in function isValidCalDAVServer() in the file CalDAVClient.php, but I am sure that this is not enough to raise a pull request, since its really just a hotfix.
Any ideas? Is this a bug of NextCloud?