Best of luck to you all who are still struggling.

Final note: For me, I don't think my browsers or credit card were the issue. I had the same problem with Chrome, Firefox, and IE. I do not use any adblock (e.g. NoScript) addons or software. My credit card works fine for FINAL FANTASY XI (to which I still have an active subscription, last charged on Aug. 30, 2013) as well as for UltimatePay.

I still don't know what caused this issue to begin with.