Dear all... problem solved.

I've created a new FTP server (based on IIS) and it works without problem.

The previous one was created with Filezilla server. 

Fortunately, the client has an FTP server based on IIS, so with this test we consider the integration in their preproduction environment will be fine.

Note: I've changed the "Server List Style" to "MSDOS" for this new server ;)

Thanks to all for your time.

Best regards,

Francisco López