I have the same issue with my Macs, but only on the newer OS. Mavericks does the Calendar and Adressbook without any issues for initial connect and afterwards.
Apple changed some inside stuff on the CardDAV of OSX after Mavericks.
Mavericks works out of the box with both (CardDAV & CalDAV).
But not anything newer for OSX.
iPhones and iPads work without any hassle, except copying the link into the config.
I mail the URL from my Mac to an iOS, then I can copy and paste it into iOS Settings for a new CalDAV or CardDAV connection.
I can’t open Nextcloud, copy the link and paste it into the iOS, it just opens up the link in a browser window.
For those wondering why such an old OSX:
Mavericks is already quite old, but still works ok - and is the last OSX Version where Lightscribe still works. The Mavericks Box (An old iMac 20") is basically my CD/DVD Lightscribe burner… 
My 2 cents
Andy