SFTP Smart Service: invalid username/password

Hi all,
we are looking for a smart service which could realize a FTPs (secure, with TLS/SLL encryption).
I'm trying to use the service "Send File Over FTP", which is part of the SFTP Smart Service Plugin. I configured all the parameters like I saw in documentation.
However, I always receive the error "Invalid username/password". Might this error be caused by a wrong type of used service?
I'm using the FTP service to realize a FTPs connection.
Thanks in advance
Andrea