i haven't slept last night because of the same error. i thought, that i've done every possible.
downloaded steam verios 2 times, installed separate vista POL version, than overwrited steam version with it.
nothing helped.

then i've tried once more time. i've deleted ALL traces of FFXI from my HDD, cleared registery and i have downloaded it from the steam for the 3rd time.

when i launched it via steam POL poped with request for an update! which were never happen before.
it downloaded 35 files, then i was able to configurate it to work with SE account.

right now i'm downloading FF updates. 21 000 files and still as slow as it be in 2008 when quited.
we will see after around 7h when it finish updates. i'm crossing my fingers