I had the same thing happen until I actually logged out. After that, It has been logging me out automatically. I have had it happen while responding to a thread, finish typing click post and I get an error message that i dont have permission to do that. I look at the top right, and I am not logged in any more. Good thing I am in the habit of hitting ctrl-c at the end of long posts in case this happens, That way if something happens, i can just log in and paste the text, and not lose it...Haven't had any issues staying logged in, didn't even have to log in after the upgrade.
I deleted every reference to tbricks in my history, cache and cookies. Re logged in, and it happened again. Today, same thing. If I am inactive for like 10 minutes(havent timed it) it logs me out. (and apparently typing in a msg window is considered inactivity.) I KNOW there used to be a "keep me logged in" setting somewhere but i can't find it any more.