Just a follow up that may suggest why this happens,

From what I've discovered today in my theory at least, IF you use a credit/debit card with an static IP Address the system will assume that IP is the origin and will record that card under that ip. On the other hand if you use a dynamic IP which most of us use, the system will record your card number with that IP Address at that given time if you pay for a sub or anything on the mogstation, in turn this will trigger the fraud detection in the billing system which the mogstation uses. So basically use a different IP Address from what you use originally from the first ever payment, and your blocked until square enix or mogstation resets the whitelist or the general database where it stores the Card number.

This system is quite old as mentioned above, and if square enix are breached from an hacker and some how get an hold of the database of your card number, ur basically screwed. So in all this can cause problems to the consumer. I also noticed some moderators try to give help on the forums about this issue:

1.) Make sure that you are entering all of your card information (CC#, expiration date, CVV#, etc) correctly. <Yes this is obviously true
2.) Make sure that your financial institution has all of your latest billing information (address, name, etc). <False
3.) Contact your financial institution to make sure there no issues with your account. < I'm guessing this is for people that use credit cards and have some outstanding fees
4.) After taking the preceding steps, if you are still encountering error code messages, please wait 24 hours and then try again. < this is bs, unless they tell us to wait 24 hours for the system to reset your IP after 24 hours or 1 month.. which is a giant waste of time and money