Nextcloud onlyoffice - file locking not working

Dear all,

I use Nextcloud (V28) and also OnlyOffice with other users. Both desktop-version and web-version.

The problem is that, when 2 people work on one OnlyOffice-document, changes get lost aka there is no file-locking or warning.

Is there any solution to this problem?
According to Nextcloud there should be an automatic file-locking since Nextcloud Hub 7. ( But that doesnt work.

Any ideas?
Best regards,