BuddyPress.org

When you installed BuddyPress, did you do that *before* you set BP_ROOT_BLOG in your config file? I think what has happened is that your root site will have Pages created for Members, Groups, Forums, etc., but BuddyPress needs those created on the BP_ROOT_BLOG site.

Try creating new Pages on the BP_ROOT_BLOG site, and then go to the BuddyPress > Components menu in that site’s wp-admin, and map the components to a page which you’ve just created.