Webdav mail merge word (serienbrief) excel does not work


I have a connection with WebDAV to the nextcloud instance and we see now that work merge mail does not accept the excel files as data source anymore.

Following message I receive:

During the opening of this document the following SQL command is used
SELCT * FROM 'Tabe1$'
Data from the database will be added to your document. Do you want to proceed

If you say yes, you can select the excel file again via windows explorer and it loads quite a while and might ask you for your credentials (nextcloud I think) and after that it doesnt work.

If I copy the files to my desktop and select the excel file as database everything works fine. But this shouldn’t be the case. I would like to offer users a normal way of doing this.

Has anyone an idea how to fix this?

Thanks for Help!

Nextcloud version 21.0.1
Operating system Debian