While I wasn't actually using bookmarks when it happened, I went over and changed all the bookmarks, logged out and then back in.

But it seems to be happening even though. I am logged in, reading a thread. The url shows https. Then I leave the browser for a while, and when I return I hit the link "MyFlyerTalk" (hoovering it displays it as https) to refresh and see new posts in subscribed threads. I then get redirected to log in page.