ProFTP Won't Return Directory Listing

You either forgot to open/forward the passive ports on your firewall or you didn't enable them in your config.

Uncomment this line in your config:

# PassivePorts                  49152 65534

Making it:

PassivePorts                  49152 65534

Then open up these ports in your firewall: 20, 21 and the range 49152-65534.
Make sure you restart the ProFTP service after this.

To understand active vs. passive FTP: link.