If during access FTP, you get the FTP error: 421 Sorry, cleartext sessions are not accepted on this server.
It means that the server is using “FTP Over Explicit TLS/SSL”
1. Add ftpes:// before the domain name for some ftp client like Filezilla
2. For FlashFXP: Go to quick connect (F8) -> SSL tab -> Auth TLS
And in general tab, you fill some field like normal