I've got additional troubleshooting steps so you can resend the invoice using a different email address, Deerchaser.

You need to update the email address through the customer's profile and resend the invoice. The same goes when changing the email on the project side. It should be done through the customer's profile.

Meanwhile, if you're getting the same error, the data on the invoice could be damage. You'll want to delete and re-create the invoice. Then, send it using the new email address of the customer.

Also, you can use supported browsers to get the best out of your QuickBooks experience. Check out this link to learn what browser you need: System requirements for QuickBooks Online, Accountant, and QuickBooks Self-Employed.

Let me know how this goes by commenting below. I'll be right here to help ensure you're able to resend the invoice.