In case you run Horde, NextCloud or other CalDAV/CardDAV server products, sooner or later you will encounter users who want to use the MacOS Addressbook application, “Contacts”, to access their server contacts. As of MacOS 11.5.2, the apple contacts app only supports one addressbook per principal. It will pick the first addressbook and use it for reading and writing. Unfortunately, the first addressbook exposed by Horde’s carddav system often is the favourite recipients addressbook. This is readonly.
There is no real solution to this yet. You can try to trick your carddav server into having the “right” addressbook as the first in the list but this is about it.
If anybody has an idea how to tackle this limitation without breaking carddav for all the other clients, please let me know.