I'm now getting this problem, too. I doubt if it's related to CAPTCHA, but it started at about the time I installed CAPTCHA a few days ago. As root, I will be logged in doing things, all is going well, and then I'll get the error message along the lines of "you must be logged in to do that". Looking around, I discover the login box is visible in the sidebar. After logging back in, all goes well for a while, then the cycle repeats. This has happened several times over the past few days, never happened before.