Somewhere along the way PSN policy must have changed. I've always used the PSN to make purchases because I live outside the US and it's a convenient way to get games in English. Suddenly, a bit before the Dark Souls DLC was released to the best of my knowledge, PSN stopped accepting my american credit card. Is this something that can be corrected or worked around?
I'm sure you already checked, but your card isn't expired right? I really didn't see any announcment about PSN stop using certain cards in the last week or two, but it's a known issue of people having trouble using their credit/debit cards.
You can go here https://store.sonyentertainmentnetwork.com/#!/en-u
Sign In > Hover over your username > Account Settings
There you can overlook and edit if your info (instead of using the XMB).
No. Sony does not intend that people that live outside the United States use the US Playstation Store, and regularly revises its card acceptance policies in order to prevent people that do that from using credit cards to fund their wallets. If the billing address of the account is not in the US, then the card will not be accepted. There is no such thing as an American credit card. The card must have a US billing address in order to be accepted.
You won't get an official reply in this forum. Sony doesn't provide Playstation customer support via forum. It never did. If you want an explanation from Playstation customer support, you have to use the phone, or live chat. Be aware that Playstation customer support has no access to the payment system, and can't tell you for certain why the transaction was rejected. There are lots of possible reasons, and as I said, the fact that it was accepted before, does not mean that it will be accepted now.