I have asked the two individuals in our organization who do the bulk of the job set up and invoicing to gather as much detailed information as possible to help further describe what is happening.  One vexing aspect is that the occurrence is intermittent, so it is difficult to pin down what is causing the error.  It may be a user action in issuing, modifying, or voiding previous invoices to accommodate customer demands for separating items that were initially invoiced together or something else like that.  

We will gather the backup we can and share it with all of you who have been kind enough to lean in to see what can be done to resolve this problem.

Thankyou.