Need help trouble shooting on UCS

I have been trying now for a long time to get OnlyOffice to work in NextCloud on UCS 5…

I keep getting this error from the interface when I try to press save in Admin Settings → ONLYOFFICE:
Error when trying to connect (Error occurred in the document service: Error while downloading the document file to be converted.) (version

I have no idea how to even trouble shoot this, especially since it’s on UCS so everything is very univentiony (f.eks. How To Edit config.php in Nextcloud App - Community - Univention Help)