Just figured out that my calendars in thunderbird where not working anymore after the nextcloud 11.0.0. A quick look and comparison I spotted the change, "caldav" changed to "dav" in the url. Example needed?
#old https://foo.bar/remote.php/caldav/calendars/user/calendar #new https://foo.bar/remote.php/dav/calendars/user/calendar
In thunderbird, you can change this quickly. Stop thunderbird and edit "~/.thunderbird/