Well, welcome to this forum - I'm sorry to hear you had problems posting here.
I have increased both the default cookie length and the session timeout to 2 hours, so this shouldn't happen again so soon. In any case: if you're going to write a really lengthy post, I'd advise you to copy (CTRL+C) the message's content before actually submitting your message (or alternatively, write the message in an offline text editor and copy-paste it here). That way you don't lose all your hard work. I know that sucks - had the same problem several times before in all kinds of different forums... now I always copy my messages, just in case, even if they're only a few sentences.
As for the page showing up blank even after going back in the browser - I wasn't able to reproduce that. The message text was gone after being asked to login again, but hitting the "back"-button of the browser for two times successfully restored my message's content. Might be a browser thing, I'm not sure
