WindowsError:5 when synchronizing some files

Hi.
I鈥檓 having a problem when one user is trying to sync some files in a folder.

All the files in that folder get a WindowsError: 5: Access is denied. The image I attached only shows PDF files, but this happens with other types of files.
If I open this folder on File Explorer, all the files have a random text after the extension.
I check user permissions on the folder, and he has write permissions. The computer has been restarted several times, and the Nextcloud client has been upgraded from version 4 to the latest version, and the error keeps showing.

What more can I do to fix this or to try to understand where the problem comes from?

similar reports in the issue tracker: Desktop client can't sync some files, says WindowsError: 5 Access denied 路 Issue #3906 路 nextcloud/desktop 路 GitHub