Did you find the solution for this? I am seeing this error message also. When I create an invoice with multiple customers (to share payment) and then try to "send" the invoice, this error message results. With only one customer, it isn't an issue. I've noticed that 2 invoices are being created, but the "create" API only returns one invoice id. The invoice ID is not visible in PayPal's manage invoices interface. I cannot find any documentation to cover this, but I believe the description is hinting at the reason. The API Responses HTTP Status Codes says this for http code 422 "The API cannot complete the requested action and might require interaction with APIs or processes outside of the current request...."
... View more