I encountered the some problem a few times when trying to book tickets at Air Asia's web site, both on the Thai and Malaysian sites.

I suggest to wait a few hours and then try again. That always worked for me.

Just make sure your reservation did not get through before you try again. You can check the status of your reservation on their web site (go to 'Bookings' then 'Manage My Booking' then 'Status Of Online Bookings'). You have to input your credit card number to check if there were any reservations with that card in the past few days.

Also make sure that your card does not get blocked. I have a BofA credit card and the first time I used it with Air Asia it got automatically blocked. I had to call the bank's prevention department to have it cleared.

Good luck.