If the login cookie expires between the 'post a message' form being sent to the user and the form being returned with the message typed in, the user is asked to login again, which is fine. The problem is that having done so they are sent to a blank copy of the posting form and their entered message is thrown away. Using the browser back button to return to the previous instance of the form does not recover the text, which appears to be permanently lost.

If you haven't done so, delete the cookie for the website and log in again.

After that - the cookie's set for two hours. If you take more than two hours to write a post, you may experience the effects above. Otherwise, you should be fine.