I had the same problem on two occurences:

1) Zoning into Castle Oztroja... same description as everyone else downloading data screen appears and then the FFXI-3001 error
This happened a while ago and I was able to move my character by logging into my character on my other PC on the same router/modem.

2) Zoning into Abyssea-Altep at first I was able to do what is stated above (use my other PC) but now I can't log into that zone from either PC and a friend must log me in if I go into those zones.

I tried everything from modem/router reset, port forwarding, called ISP, checked files and reinstalled the whole game. Nothing worked.
So now I avoid both these zones... but its frustrating if you have a quest involving one of these zones