I appreciate the detailed information and the troubleshooting steps you did, BGC.
This error can occur if the Microsoft Outlook components are damaged. Its compatibility with QuickBooks or your operating system can cause the error to occur, too.
We can repair Microsoft Outlook so you can send your invoices again. You can use this article for the detailed steps: Repair an Office Application.
You can also browse this article if you need more troubleshooting steps: QuickBooks is Unable to Send your Emails to Outlook.
Let me know if you need anything else.