The odds are your cookie has been corrupted. Manually clear the cookie that the forum has given you (Google 'clear cookie' with your browser type as to see how if you need) and then log back in and you should stay logged in.
1) check to see if your OS is up to date
2) switch browsers (I like Google Chrome), also make sure the browser is upgraded to the latest version
3) update flash player
4) delete all browser cache and cookies
5) delete flash cache ( done HERE )
click the "delete all sites" button.