Have you been able to test FTP access within your local network using an FTP client? Are you able to access other FTP-enabled devices on your home network using the same method?
Looks like your router doesn’t support Passive FTP ALG – (“Application Layer Gateway”) so it’s not translating that PASV instruction to use your outside address.
Some routers do, some don’t – and finding documentation on which do and which don’t is darn near impossible.
Only option I know of for you is to modify the FTP configuration in the Duo to specify your outside address… and that’s got its own problems if your outside address changes…
The FTP works fine when I am at the local network. The strange thing is that before I had a WD Mybook World Edition 2 and the FTP worked ok when I was outside with the same router (Thomson TG585 v8).