ACH Echeck transaction rejected immediately

When I try to receive payment from a customer using an echeck with Quickbooks Online, the transaction fails immediately with the message "The transaction was declined because the bank account is incorrect, or there is a security concern".  It shows up on the Quickbooks Payments transaction report as Status:Declined and Response:Transaction Declined.  I know the routing and account numbers are correct because we've processed payments before.  I've also tried using a completely different account owned by the same customer and the error is the same.  The customer says that his bank never even saw the attempt.  Please help!