OnlyOffice does not synchronize

Windows 10 | nextcloud 19.1

Hi,
I installed the app OnlyOffice.
I changed an excel sheet on the server (browser). Than I tried to synchronize this excel sheet (same with word) with my desktop client. Nothing happens. It seems as if the date stamp does not change, when the excel file is changed.
Do you have any idea what goes wrong here?
Thanks.
Regards, Thomas