Got a strange GMail message that persisted with a single message. I composed an email, and clicked on send. Up popped the following message:
Oops... a server error occurred and your email was not sent. (#793)
It persisted. I was able to create and send another message, and yet when I went to my drafts folder to send it again, I got the error again.
I finally solved it by closing the browser and re-signing into GMail.