Nextcloud error after migration from NS7

Yes, because there’s already an account provider on the node.

Maybe it helps to restart the services?

runagent -m nextcloud1 systemctl --user restart nextcloud.service nextcloud-redis.service nextcloud-db.service nextcloud-app.service nextcloud-nginx.service nextcloud-notify_push.service

or

As regards nextcloud-redis, you could try to remove the volume as shown here: