How To Fix “421 Cleartext Sessions Not Accepted” Error In Filezilla?

Server is enforcing TLS encryption for the connections. Hence, need to add prefix ftpes:// before the hostname of the server

if trying to connection to example.com,

simply change the hostname to ftpes://example.com

Refer to this article for more details if required. 

http://digitizor.com/2011/03/02/fix-421-cleartext-sessions/