I was able to solve it myself.
By editing /etc/loolwsd/loolwsd.xml I was able to use Collabora from both ownCloud and Nextcloud.
I wrote the following in the storage section:
<host desc="Regex pattern of hostname to allow or deny." allow="true">127\.0\.0\.1</host>
<host desc="Regex pattern of hostname to allow or deny." allow="true">owncloud\.myserver\.com</host>
Thanks,