Have you ever had problem with such error while connecting to the FTP server from some private network? I faced this problem and I knew it is something connected with the firewall ( debian linux server and IPtables ) but I didnt rememebr what was that...

 I had RELATED and ESTABLISHED packets allowed in the firewall but FTP worked in neither pasive nor active mode.
It started working after running ip_conntrack_ftp module by command:

modprobe ip_nat_ftp

 You can find more information about FTP connection, cause of this problem and solution here:

http://linux-vashi.blogspot.com/2007/12/ftp-error-500-invalid-port-connection.html