"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:":
You'll probably need to use another FTP client besides your default browser.
I don't know if newer versions of IE support FTP over SSL.
Filezilla might be a good choice:
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