Yeah it happens to me all the time and I have no clue why. Even though I specifically checked
Always stay logged in box, still the same...
@ irradiated_1 - When that happens it makes you wanna scream, right ? It is so ANNOYING... in the lack of better word. I fixed (temporary) my problem like this. When that happens and you have to sing in again, just do that. Sign in again in that window. Now, when you are signed in, your message windo will be completely empty... right ? yes... do not worry. Just press BACK button in your browser (I am not sure for other browsers but it works in IE) and voila. There will be old message window with your message inside. Just copy it (CTRL+C) to clipboard, and reopen that window and paste your message in it (CTRL+P).
I haven't tried to simply repost it from within that same window, maybe it would also work but I didn't want to take chances. Once you have your message copied to the clipboard, it's easier to open new post window, paste the message and nicely send it from the scratch... it always work on my end

Cheers !