[Bug]: Directory Symbolic Link _and_ Junction from a Nextcloud share to another Nextcloud share is synced on Windows, with errors

[Sieboldianus]

⚠️ Before submitting, please verify the following: ⚠️

• This is a bug, not a question or a configuration issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server and Desktop Client are up to date. See Server Maintenance and Release Schedule and Desktop Releases for supported versions.
• I agree to follow Nextcloud's Code of Conduct

Bug description

With the latest Nextcloud client on Windows 11, I started seeing upload errors with "unknown" in the client. After inspecting, I traced the errors to a Directory symbolic link I created from a share within my Nextcloud sync folder to another sync folder. I tested with Junctions and saw the same effect.

Steps to reproduce

1. Create a NTFS Directory symbolic link with mklink /D from a folder that is synced to a Nextcloud server to another folder that is also synced to the same Nextcloud server

mklink /D C:\Users\user\nc_share1\junction_folder C:\Users\user\Nextcloud\nc_share2\junction_folder_target
symbolische Verknüpfung erstellt für C:\Users\user\nc_share1\junction_folder <<===>> C:\Users\user\Nextcloud\nc_share2\junction_folder_target

The same applies to Junctions:

mklink /J C:\Users\user\nc_share1\junction_folder C:\Users\user\Nextcloud\nc_share2\junction_folder_target
Verbindung erstellt für C:\Users\user\nc_share1\junction_folder <<===>> C:\Users\user\Nextcloud\nc_share2\junction_folder_target

3. Observe Nextcloud attempting to upload files again
   

4. Observe Nextcloud to fail after scanning files with "Unknown Server error"

• after repeating with Junctions/Hardlinks, I sometimes saw a few files being actually synced to the server; but it always fails after a few files (<100).

I have both folders as individual shares:

Other observations:

• There's no error on the Nextcloud Server.
• The folder on the server is also shared with many different users and contains a lot of files (3000, about 15GB). I don't think this is the cause.
• If I create a shortcut (*.lnk), it is not synced (only the *.lnk file is synced, as expected)
• I searched and found #4770, which is also related to Junctions. This indicates that the client may have some unidentified issues with Junctions/Hardlinks.

Expected behavior

Directory Junctions are not synced, as is documented.

Which files are affected by this bug

ExpNature1to5.pdf

Operating system

Windows

Which version of the operating system you are running.

Windows 11

Package

Official Windows MSI

Nextcloud Server version

28.0.9

Nextcloud Desktop Client version

4.14.0

Is this bug present after an update or on a fresh install?

Updated from a minor version (ex. 3.4.2 to 3.4.4)

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

Are you using an external user-backend?

• Default internal user-backend
• LDAP/ Active Directory
• SSO - SAML
• Other

Nextcloud Server logs

No response

Additional info

No response