BigCommerce site: PayPal Website Payments Pro generating error message

Website is: [login to view URL] Any attempt to make a purchase with a credit or debit card, results in this error message: Your financial institution has indicated that it could not successfully authenticate this transaction. To protect against unauthorized use, this card cannot be used to complete your purchase. You may complete the purchase by selecting another form of payment or use a different card. (end of error message). We have tried many different credit cards and debit cards, in the USA, using the correct billing & shipping addresses (same address to keep it simple), and knowing that we are using good cards and all the correct information: We know that the problem is not the cards or the financial institution rejecting the transaction for some reason. SEE THIS FORUM POSTING about same problem we are having: [login to view URL] You can conclude from all the postings online that this problem is a general fault/defect of BC and PP Website Payments Pro integration. We need a real expert to solve this problem. If we post a ticket on BigCommerce support, they will say it is the API credentials entered wrong but we know that is not true: We know that the PayPa; API credentials are entered properly. We will give you access to the paypal acccount so that you can verify this, if you wish. We also investigated this as a possible cause of the problem: [login to view URL] We attempted to fix the problem by utilizing his recommendation, but it also didn't fix the problem. PayPal Express Checkout is working fine.