Have you got 2 computers hanging of the router ?
If so try ftp'ing to the Amigas Local address (The one assigned by the router) instead of going out over the internet using the routers ip and back in via the NAT translation.
If that works ok then its something odd in miami or your router
Got the same version running on my Amiga at the Moment, with port 21 pointing at it.
Works a dream for me... Excellent FTP proggy
