Oh dear - that error a lot of people have had sadly and unfortunately doesn't have an easy fix as there are several causes to it (one possible cause is the bank that issued your card is mistakenly blocking Square Enix from authorizing it, thinking it's a fraudulent access when it's not.

I'm not sure how you can fix this issue - I've heard people actually gave up trying to use their cards to pay for FFXIV's subscription because of this error and just went with another payment method like a prepaid time card, so I'm not really sure how to resolve this. I think you might have to contact Square Enix's Support Hotline, but just be aware they might take a while to resolve this issue.

I'm really sorry I couldn't have been of more help.

EDIT: I forgot Square Enix had this thread on the subject in the support section which might help: http://forum.square-enix.com/ffxiv/t...de-401-and-601