FileZilla Server 425 Can't open data connection for transfer of "/"

You have most probably not opened the data connection ports on the firewall of the server (or have not forwarded the ports on NAT, if there's any).

Check what ports the FTP server is using and open/forward them.

FileZilla Server Options

Another problem is, that you have not configured a correct external IP address in the FTP server.

The FileZilla client can workaround that:

Server sent passive reply with unroutable address. Using server address instead.

But it's just a heuristics, that not all FTP client will do. So make sure you fix this too, even if it is not the immediate problem you have.

See the External Server IP Address for passive mode transfers section in the above screenshot.

See my article on network configuration for FTP for some background.