I had this error recently because my DNS gave me the internal IP of the Onlyoffice host and this needs to be allowed in Nextcloud.
From the wiki (to share the solution here too):
“violates local access rules” error
add
'allow_local_remote_servers' => true,to /usr/share/nextcloud/config/config.php.
Maybe your DNS provides a local IP address for the Onlyoffice host. This way you can allow local network addresses.