Payment not processed

I thought I'd give Zoho Invoice a try. I only setup one customer to gauge their experience. Problems.

When they try to make their payment, it says "cannot process". On my side, I get no notification other than "unable to process". I am using PayPal Pro. It must be setup correctly, as I do not see any error notifications. My client is baffled and can't provide enough info for me to solve the problem, and I don't have ANY information on my side to solve the problem.

Any suggestions or ideas?
(and yes, my client, a successful doctor, has enough credit to pay the invoice).

As a new user trying out Zoho, I am disappointed they don't offer support. Not a comfortable feeling letting them handle such important aspects of my livelihood and those of my employees/contractors.