The first issue that you have described is that the Document Server cannot access Nextcloud and download the file from the storage.
One of the possible solutions for CentOS and derivatives is the firewall exception:
(do not forget to change the port)
sudo firewall-cmd --zone=public --add-port=80/tcp --permanent
sudo firewall-cmd --reload
Also you could try to download the latest version of our connector. It was released few days ago. Unpack it to the /nextcloud/apps/onlyoffice/ => open the NextCloud config file /nextcloud/config/config.php => and add a section to it:
‘verify_peer_off’ => TRUE,
The second issue is connected with the Collabora Online plugin. The Collabora Online plugin works incorrectly and prevents our editors from being started by default. Please turn off the Collabora Online plugin to use ONLYOFFICE by default.
Looking forward to hearing from you.