Correct me if you've already done it, but have you gone to the old square enix account management site
https://secure.square-enix.com/accou...n?cont=account and tried to remove the credit card or update the information of your credit card here AND THEN trying to wait and try again at the mogstation? I have a feeling it might have saved some wrong info or kept it there thinking that you already have a credit card to it and giving you weird errors. Some people have had success with this on reddit and I think I saw a post about it here. So the process is:
1. Log in to
https://secure.square-enix.com/accou...n?cont=account
2. Go to Regist/Update Payment Information Tab and either remove CC or Update CC there
2. Try purchasing at Mogstation
3. If you continue having errors, try waiting 24-48 hours and try again.
Again, some people has had luck with this.
I am myself a 1.0 non-legacy and have no issues subscribing. I didn't subscribe to 1.0 and only bought the game so there was no credit card registered on the old square website. I hope you guys get it worked out.