I use the recommended sync client DAVx5. After some hickups basic sync of contacts works.
However, contact pictures are not synced from phone to cloud.
There is an old thread from 2016 stating it basically works: Contacts app - sync contact pictures
However, here, no contact pictures are synced. When a contact is changed on the phone, I get the “broken contact” flag from Nextcloud, see Contacts app - sync contact pictures which is closed, but not fixed.
Any idea what can be the problem with the pictures?