The folder/file names are stored on the NAS by using unicode but some FTP clients, included Internet Explorer/Windows Explorer, don't support unicode.  To solve this issue, please: 

1. In “System Settings-Language Setting” page, select the language that most users use.
2. Go to Network Settings-FTP Service page and disable Unicode support.

However, it is highly suggested to use FTP clients with unicode support, for example, use Filezilla. 

Applied Models: All NAS Series