Hi mfrey,

The status of this investigation is still In progress and the product engineers are still finding a fix for this because they state that this appears to be an Outlook error. 

As of now, the workaround available is to download the latest Office 365 update. You can refer to this article for the steps: Outlook cannot log on when emailing in QuickBooks Desktop.

You can also send your invoice to your customer via e-mail by saving your invoice as a .pdf file in QBDT and then, attach the invoice in the e-mail.

Also, I suggest contacting QuickBooks Desktop Care Team to add you to the list of affected users so that once this issue is fixed, you will be notified right away via e-mail.

I appreciate your patience in this matter while our engineers are working for a fix. Take care.