Just caught it. In a forum, thread open, reading, got sidetracked, came back, swing cursor to check alert on another thread, "You must be logged into do that." Out in main forum, doesn't seem to be a problem.
Did you check the checkbox "stay logged in"? Technically it probably logs you in again once you are logged out, but it should help. If it does not help, you might have an issue with cookie handling.
Haven't had to do that for the most part. Go off, run errands, take a nap, and come back and everything's copacetic. Seems to happen when I'm "deeper" in the menu (not all the time). What provoked the post was that I was getting 15-30 seconds to put together a reply --- this reply, I'm dawdling with just to see if I can get it to bite. Could be my browser for all I know --- it's been getting a little weird off and on from "update" to "update." Here goes nuttin'.