I had this issue when I wanted to buy miles some months ago - rejected my credit card each time I tried. Finally called customer service and they sighed, and said "I will record this now. Are you ok ? Fine, then tell me now your credit card data".
From what I saw when the new window for the credit card data opened, the miles purchase seems to be processed by a bank (another one than the normal ticket purchases ?) - so maybe they have some issues with some of the credit card data ?
I have a foreign credit card, and sometimes also had issues in shops in Athens.