Onlyoffice Error while trying to connect


i’ve a problem with a fresh install of nextcloud and Onlyoffice Docs Community Edition to edit files.
When i try to save my onlyoffice server address (the name, not the IP) in the onlyoffice admin page of my NC portal, i’ve this message :

“Error while trying to connect (An error occurred in the document service: Error while downloading the document file to be converted.) (Version”

My setup is composed by two servers, both Ubuntu 20.04.2 LTS
On the NC server, i’ve installed the latest version 20.0.9 and on the onlyoffice docs CE server the one.
Boths servers are on the same network.

For testing, i’ve try to connect on the public onlyoffice demo server and i v’e the same problem and the same message (exclude the version at the end of the message differ)

I’ve tried to use the NC appliance and connect it to my onlyoffice docs CE and it’s works. So i suppose the problem should come from my NC install. maybe a specific module ? but which one ?

Firewall on my servers are disabled.

I followed this procedure to install my NC server : Example installation on Ubuntu 20.04 LTS — Nextcloud latest Administration Manual latest documentation

i followed this procedure to install onlyoffice docs CE : Installing ONLYOFFICE Docs for Debian, Ubuntu, and derivatives - ONLYOFFICE

Thanks in advance.