I think he was asking you to test the connectivity between the Nextcloud and the onlyoffice machines… but from what I know it is not a simple connectivity issue. It seams that the NGINX service is dying and it can not meet the connection request from the Nextcloud machine. I intend to put cronjob to restart only the NGINX service instead of the whole machine and see how this goes for a week.
This idea of mine came from here: