I believe is the system issue between AirAsia payment system and Amex in Malaysia. I have a lots of similar experience when making payment for online vendor, such as Japan based vendor and US amazon where they have no issue of accepting foreign credit (which has been successful in past years), but I have been unable to make any payment using all foreign credit card since past few years.
But yes maybe you should give Cebu Pacific a try. Have seen some not bad review on it too