501 FTP Reply Code - KB Article #1526Related Articles --
501 Syntax error in parameters or argument.
A 501 reply code may be sent in response to any command that requires or supports the optional use of a parameter or argument. It is a permanent negative reply, which means the client is discouraged from sending the exact command and parameter(s) again since the server will respond with the same reply code. It differs from the 500 reply code in that the server recognizes the command present but is unable to take the requested action due to a syntax error in the parameter(s) present with the command. Sending the same command to the server with a corrected parameter may result in a different reply code.
A 501 reply code may be sent in response to any command requiring or supporting the use of a parameter or argument.