This is not our product. It’s a fork of one of the old versions of Document server. Nextcloud team created it. We have information about it here: Community Document Server app in Nextcloud 18
Please install Document server on the separate clean server. You can find guides here:Installation Guides - Docs Community Edition - ONLYOFFICE
After that use connector app ( ONLYOFFICE Version 7.3.0) to integrate it with your Nextcloud portal.
After that please re-check the issue.