I had the same problem as many others in this discussion: owncloud server crashed, contacts on the phone only, no way to reconnect to my new nextcloud server. My work-around: I used MyPhoneExplorer to backup my contacts and restore them to the new account.
Roughly those were my steps:
install MyPhoneExplorer on notebook and the MyPhoneExplorer client on the Android phone on the phone select the DAVdroid account for synchronisation backup all DAVdroid contacts from phone to my notebook delete the not working DAVdroid account (which deletes all my contacts as well) create a new DAVdroid account pointing to my new Nextcloud server finally restore the backup from the notebook to the phone