Are there any errors in the OnlyOffice app instance logs?
I got the same error and for me it was a permission problem, installed the app from within Nextcloud, I had the following in the logs:
Error documentserver_community OCA\DocumentServer\Document\DocumentConversionException: sh: ./x2t: Permission denied
Which was fixed by:
- $ cd /var/www/vhost.server.domain/htdocs/apps/documentserver_community/3rdparty/onlyoffice/documentserver/server/FileConverter/bin/
$ chmod u+x docbuilder x2t