I was having the same problem, and just managed to fix it. It looks like you are doing the same thing I was. In your sidebar.php file, you are displaying a list of your most recent posts by calling $posts = get_posts(). The variable $posts is used by the WordPress loop, so using it here messes up the main loop.
All I had to do to fix this problem was to rename this variable (I changed mine to $recent_posts). Hopefully this fixes your problem as well.
