calDAV cardDAV connectivity

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I can add my Nextcloud Account in Prefs, and I will see my files in Dolphin or in Konqueror. But Phonebook will not sync any contacts. And in Merkuro Calendar and Merkuro Contacts I can set up DAV connectivity but it will throw an error.
Are there methods to make Nextcloud connectivity a little more useful, or is DAV mostly disfunctional these days?

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still not working. Merkuro asks for credential many times but throws an error message when finally trying to connect.

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nobody using Contacts or Kalendar or Merkuro?

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with Beta 15 RC5 I can set up a Nextcloud account, but adresses will not sync to phone book and Merkuro does not connect at all.