The main cause of 421 Too Many Connections from this IP error in FileZilla is non-active FTP connections not being closed before opening new ones. This error happens because shared hosting providers limit simultaneous FTP connections in order to prevent server overload.
The recommended solution is to get in contact with the server administrator and encourage him/her to configure the server correctly. But if you don’t want to do that, there is another to correctly configure the server. If you get a 421 Too many connections from this IP *number* error, the FTP server is restricting your number of connections. Some servers even