We're sorry you've encountered this rare issue. We use PayPal Website Payments Pro for our credit card form, and it is possible your financial institution is trying to protect you from fraud.
In cases like this, sometimes if you check out using the standard PayPal option, rather than our credit card form, it will work (you don't need a PayPal account to do this).
If that doesn't work, then the best thing to do is to contact PayPal customer support - in our experience, they're quite good, and they'll be able to tell you if you need to contact your financial institution to "whitelist" a purchase from our site. If that happens, this information might help: