Proftpd illegal port command

Hi,
I’ve configure proftpd with virtualmin.
I have this issue.
If I try to connect (internal network device) with winscp (FTP port 21 with explicit encryption) it works correctly.
If I use an external network device WinSCP returns this error: “Illegal port command”.
PS. Virtualmin is behind firewall watchguard where I created port forwarding external port 8021 to internal port 21

Can someone help me?
Thanks