@armadillo Synchronizing events (and tasks) is a very complicated task and it’s not done with simple copy operations. All the complex operations and requirements are best covered by a separate protocol – as CalDAV.
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