Do you use a valid cerificate?
In the nextcloud onlyoffice app settings there has to be trailing slash in the “Document Editing Service address” like https://domain.tld:8082/
You may try signal-event nethserver-onlyoffice-update to reconfigure the module.
Next steps would be to reinstall documentserver or drop the db. Sometimes the documentserver rpm can’t create some links.
https://wiki.nethserver.org/doku.php?id=onlyoffice#reinstall
Is there nothing in the logfiles?
Which documentserver version do you use?
EDIT:
You may try the new module version.