Has happened to guildmates. Seems like if your first credit card doesn’t work, there’s some limit of tries before they shut down all buying for that player.
I found that sometimes when I log in, if I go to the Black Lion trading it won’t show my Gems, when this happens I also cannot use my credit card or buy anything at the trading post, I usually relog to sort it.
This might be similar to what you are experiencing?
Just wondering if someone else got the same problem recently