vsftpd under Debian sarge

Hi there,
since a few hours I'm trying to make vsftpd working behind a firewall
configured for NAT.

The FTP server is up and running with local user access enabled, and I
can login from a windows machine in the same network of the linux server
(192.168.0.0/16 private network).

The issue appears when I try to login from a machine on the external
network (internet), which reaches the linux server from the NAT IP
address configured in the firewall.

This is the response of the windows client trying to connect to the NAT
ip address:

C:\>ftp xxx.xxx.xxx.xxx <-- public NAT address
Connected to xxx.xxx.xxx.xxx.
220 (vsFTPd 2.0.1)
User (xxx.xxx.xxx.xxxnone)): fabio
Connection closed by remote host.

The vsftpd daemon is linked against libwrap but hosts.allow/hosts.deny
files are empty and, also trying to configure some parameters related to
passive mode in the vsftpd configuration file (vsftpd.conf -
pasv_promiscous, pasv_address ecc), the issue still remains.

vsftpd's log file does not report any error and no debug/verbous
settings seems to be available in the configuration file. Also other log
files listed in /var/log directory do not say anything relevant (already
issued a grep -ir ...).

Actually I'm not sure where the bad settings keep on (debian or vsftpd
side).

Any help greatly appreciated.
Thanks in advance,

Fabio