We are having the same issues with our clients too so interested in any response.

They confirm the issue and are working on fixing it...

I'm hacing the same issue.  Apparently it has been happening for months and I only noticed it now when a client confirmed that they hadn't seen an invoice.

I had an unhelpful help-chat with quickbooks today where they asked the usual things like try a different browser or send to a different email (which obviously are not the problem).  

No acknowledgment that this is a known issue, so I'm glad to see others saying it is.

Aha, I fixed it for me at least.  It was some permissions/authorization issue between gmail (what I was using as my send-from address) and quickbooks.

What I did:

1) Create an invoice and fill out whatever is necessary (you won't actually be sending it)

2) Click "review and send"

3) On the review page, click the pulldown menu for "from" and select "remove gmail address"

4) Now click the pulldown meny for "from" and select "add gmail address" to re-add your gmail

5) follow the steps that come up to authorize you gmail account

I hope that helps others