Nextcloud
Your web server is not properly set up to resolve “/.well-known/caldav”. Further information can be found in the documentation 27.
Edit File:
/etc/apache2/sites-enabled/default-ssl.conf
Add lines between </VirtualHost> and </IfModule>:
Redirect 301 /.well-known/carddav /nextcloud/remote.php/dav Redirect 301 /.well-known/caldav /nextcloud/remote.php/dav