Moved from an unrelated post to its own thread.
This was a session timeout. There is an Idle Timeout configured. It is based on HTTP GET and POST methods to pages. When you draft a response, you are performing a GET to the page, however the draft is being written locally. Then once completed the 'Post Reply' is a POST. If the time between the GET to draft a response and the POST to submit the response exceeds the Idle timeout, your session would timeout before the POST was submitted.
This is expected behavior. In a lengthy post, I sometimes draft it in 'Notepad', then do a copy/paste to submit.