I am also experiencing errors when sending a reply. I can confirm that for me the original message in this thread was present in 11.0.0 and no longer occurs.
However when I want to reply to a message that works when I navigate from the messages overview to a specific thread. When I refresh the thread page (i.e. <url>/members/demo/messages/view/1/) and try to send a reply, buddypress-message.min.js throws the following exception:
Uncaught TypeError: e.has is not a function
I tested it on a clean wordpress install without plugins with the theme twentytweny.
