3201003 - Error: "Invoice XXXXX, cannot be sent to the external system because a connection failure has occurred"

When a user goes to submit an invoice in the buying application, they receive the following validation error:

Invoice XXXXX, cannot be sent to the external system because a connection failure has occurred

"cannot be sent to the external system because a connection failure has occurred" , KBA , BNS-ARI-INV , Invoicing , Problem