I had this problem, too. Error 401 and 601. My boyfriend and I tried to pay with the same credit card. For him it just worked fine. I got the errors.

Square Enix asked me via Email to send them a picture of my credit card and make sure that the first 12 digits of the card are blacked out. You then have to send it to sel-id@square-enix.com including your ticket number because the standard email-address filters out all the attachments.

I sent them the picture and today (4 days later) I got an email that my problem is fixed and I can register and use my credit card for subscription.... and now I'm back in game.

Hope this helps you guys and solves your paying problem, too.