426 Connection Closed; Transfer Aborted or 503 Bad Sequence of Commands

Error: 426 connection closed; transfer aborted or 503 bad sequence of commands

Some servers do not allow passive transfers and will return this error if you try using passive mode. Try disabling passive mode by going to File > Manage Servers, selecting the server profile you wish to use, and unchecking the Passive box. If after doing that you receive a 425 error, try disabling any local firewalls before connecting.