We appreciate the responses. We have checked with PayPal, and they are very vague about just what would cause this error. Of course I realize they can't give information about the customer's transaction to us, but was hoping for a reason for this particular message. Here's the response we received:
To provide one of the safest online payment services, PayPal reviews every transaction before it is approved, and occasionally, we must limit funding options or decline a payment. These decisions consider every aspect of the transaction, including the merchant's settings and PayPal's statistical models. From time to time, these complex security measures may affect some accounts in good standing.
They have suggested we have the customer contact them about the specific incident. We will see if the customer will do that, but the customer is a bit turned off by the hassle involved, since their card works on other merchant's sites.
As to having the card associated with a PayPal account, the customer indicates that is not the case. I've seen that happen, but the message then indicates the problem is because the credit card is associated with the PayPal account, and they must either use another credit card, or log into their PayPal account.
Thanks for the suggestions. If we do find the actual cause, we'll post it here.