421 Sorry, cleartext sessions are not accepted on this server.
Some times when you connect the webhost via FileZilla you may encounter this error message, because the FTP SSL encryption option will be incorrect.
Solution:-
Main >> Service Configuration >> FTP Server Configuration
TLS Encryption Support >> “optional”
On the FTP client use server type as
Host: domain or Ip
Port : 21
Server Type : FTPES -FTP over explicit TLS/SSL
Enable the passive port range for Pure-FTPd
vim /etc/pure-ftpd.conf configuration file
enable the passive port range
PassivePortRange 30000 65000
Save the pure-ftpd.conf file and restart FTP service.
Enable the passive ports range in your server firewall
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Reference : http://docs.cpanel.net/twiki/bin/view/AllDocumentation/WHMDocs/FTPPassiveMode