Facing below issue.
Thats normal. There needs to information in the message box. It can't be blank to post a reply.
I am facing the same issue on the Public Facing form when posting a response. The difference is the user does have a message in the response, yet receiving the error.
Seeing the rest of the page would likely be helpful.
Also my first guess would be that you installed a 3rd party theme mod that you should remove and see if doing so fixes the issue.
