I have had this problem as well and its always with my visa credit card. i can use that card anywhere else and there is never any issues, only SE is the one that gives me errors when I try to use the card. I have sent in multiple tickets on the issue but they are always ignored. I have talked to my bank on multiple occasions as to why I get an error when trying to use my card for here but they never see any issues on their end. Most likely SE doesnt even know why the error pops up though it would be nice to hear a reply from them on the issue.