On my “coming soon” homepage the sidebar is showing, despite the fact that the settings are set to “templage: blank – no header, no footer” and “layout: no sidebar:” I’ll post the url in a private reply.
You can try deactivating all active plugins, flush settings if you are using any caching plugins and check if that helps. You can try on another computer as well. But it should not be a problem if it is seen only when logged in? :)