Cleartext sessions are not accepted on this serve
“421 Sorry, cleartext sessions are not accepted on this server” means that the server is using “FTP Over Explicit TLS/SSL”. This is a Good Thing.
You might be able to fix it by giving the protocol “ftpes:” instead of “ftp:”:
ftpes://ftp.somesite.com/some_folder
… or …
ftps://ftp.somesite.com/some_folder