Hi there, Mir7.
Have we tried sending another invoice and still getting the error although the invoice was sent? If we're getting the error and the invoice wasn't sent, our engineer are currently working on the issue.
Our engineers are currently looking into the delivery error when sending invoices. I suggest reaching out to us so we can include your account to the notification list.
In the meantime, we can download the invoice and manually send it outside QuickBooks. You might want to check this article when its our time to receive customer payments.
Let me know if you have other questions. Thanks!