I, too, have been getting the invoice bounced back for no reason.   This need to be fixed ASAP!   My alternative solution is to click on the action with the gear next to it while looking at the invoice page.  click on the arrow below and select "share invoice link", copy it then paste it to your regular email account...yahoo, gmail or whatever.  

This is ridiculous that we have to do this.   We should all receive a few free months of QBO for our inconvenience!!