First time I have experienced it too, June 28th. I compiled the invoice using the ipad app, sent it and all appeared fine. I only noticed once I logged into the browser (which gives SO many more details and options) that it showed me the error. Weirdly I tried or two things which didn’t work, then tried editing and resending (adding in a missing cc) and it sent fine. Very strange and can imagine very annoying for those who are stuck with this problem. I did not report it but I will if it happens again.