As our hosting is complying to PCI-DSS, FTP without encryption (which transfers plaintext passwords) is not allowed.
We have thus implemented support for FTP-S with SSL/TLS on standard port 21.
FTP-S with TLS/SSL is different from sftp (and we do not support sftp) and also different than FTP-S with implicit SSL.
Please make sure that your FTP client supports FTP-S with SSL/TLS and use it in that mode. FileZila is one of them that we tested. On Mac, Yummy FTP, Transmit and CyberDuck are tested too.