vsftpd error "500 oops syntax error, command unrecognized"

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http://serverfault.com – Really strange that my FTP client receives a "500 OOPS: Syntax error, command unrecognized" when trying to upload a file to Vsftpd server after a LIST operation... Here goes the details of this error: My FTP client, let's call it ftpClient, firewall is opened for ALL connections from ANY remote machine at ANY port (it is located in an internal lab, so no security issue). My FTP server, let's call it ftpServer, is a Vsftpd FTP server. Below is the connections between the client and server from TCPDUMP output: ftpClient->ftpServer TCP 60 53056 > ftp [ACK] Seq=1 Ack=1 Win=32768 Len= (HowTos)