How do I delete or deactivate this Addressbook?
You need to go to the Contacts app in Nextcloud and then, in the bottom left there is an icon for the settings. There you can remove the recently contacted addressbook.
No, you can’t. Nextcloud failes to delete the addressbook. You can’t even delete the addresses stored in this addressbook.
I second this. It is not possible to delete ‘Recently Contacted’. Any suggestions on how to fix this? There are rumours that this might break my Sailfish OS integration with Nextcloud.
This seems to be the solution Contacts: How to delete "recently contacted"
…and that also fixed my Nextcloud integration on Sailfish.