I contacted support center concerning error 601 as well and this is what I've got:

"If you get error 601 when registering your bank card, please ensure all the details in your Square Enix account (name and home address) match the billing information of your bank card. It is essential that this information is identical."

So thats why I couldnt use somebody's else cards however I was receiving this error with my own card as well. I suppose you are in the same situation but perhaps this is where your issue lies~ If no then maybe just go back to C&B?