From another "solved" thread:
Hello,

If you are receiving an error 601 or error 401 when attempting to update your payment method, then there is an issue where the financial institution is declining the authorization attempt. Unfortunately, as SQUARE ENIX is not the financial institution, we are not provided with any information as to why the authorization was declined, only that it was. Please confirm that all billing information is entered correctly on the SQUARE ENIX Account in question. Please also note that once an authorization has been rejected, that form of payment cannot be attempted again for a full twenty-four hours, and any further attempts before that time will automatically be rejected. If you would like further information about this specific authorization attempt, please contact your financial institution. If you wish to begin playing immediately, please choose a different payment option and payment method, available through the SQUARE ENIX Account Management System.

Regards,
GM Bublex
I am recieving a 401 error code, using a card that I have used for many, many MMOs, including FFXI.

The system does not ask for my account name (I am not sure if this is a requirement), and does not trigger the securecode system used by my bank (which, again, FFXI did). As always, it makes me baulk to have to fight people to give them my money.

What is going on?