The current version of Virtualmin ships with an issue for ProFTPd on CentOS due to two modules that it needs to track connections not being loaded. There is a quick fix for this, you’ll need to edit the iptables-config file located at:
[root@pluto ~]# nano /etc/sysconfig/iptables-config
Add the end of the configuration file, add the following line:
IPTABLES_MODULES="ip_conntrack_netbios_ns ip_conntrack_ftp"
You can now restart the iptables service with the following command and FTP should work through passive mode:
[root@pluto ~]# service iptables restart