FTP access INVALID PORT COMMAND

Hi everybody. We need to access from our internal network to an external ftp server, but we can't do it because of an "invalid port command" problem. Our internal network is connected to internet via a suse 9.2 linux. I have already installed both modules ip_conntrack_ftp and ip_nat_ftp but it doesn't still work. Last week we coould access to this ftp server (of course, it works in non-passive mode) but since last thursday we can't access. Any idea? Thanks in advance. Alberto