I have the same issue using Nextcloud 11.0.1 and macOS 10.12.3. Nextcloud is installed on shared hosting. I have not yet tried synchronizing with Thunderbird and Lightning, as I would rather use the builtin calendar.
Edit: The solution at https://docs.nextcloud.com/server/11/admin_manual/issues/general_troubleshooting.html#troubleshooting-contacts-calendar worked for me