How to add remote Windows share as external storage

We have an Owncloud 9.0.1 (currently being migrated to NextCloud) Server running without any problems. For a section of the organisation we would like to securely link to their remote Windows file system via the OwnCloud remote storage system.
That means the remote office needs to edit documents locally on their windows network and other remote offices need access via ownCloud/Nextcloud. Using sftp works fine on systems where the remote sftp server is Linux based. Linking to a Windows system causes synchronisation problems, where the ownCloud users see only parts of the file structure of the Windows file server. The transfer needs to be encrypted, which eliminates an smb connection, i think.
Windows Server 2012 R2

Does anyone have a similar set-up that works without problems, and could share the detail of the windows config?

Which SFTP server software do you use here?

Can you clarify what “see only parts of the file structure” means? Does this mean they persistently see always the same set of files or do they only miss to see newly created files?

Also the output of “occ files_external:list” would be helpful.

| 4        | /xxxx  | SFTP    | Username and password | host: "xx.xx.xx.xx:22222", root: "\/", user: "xxx", password: "***"                | enable_sharing: true | user1, user2 | groupxxx          |

A, seemingly arbitrary, sub selection of the file structure is shown, and does not get updated when changes are made to the structure from within the widows network.

Bitvise SSH Server
For the access rights on the file server Mircosofts “Access Based Enumeration” is used.