<ui:include src="/sections/login/sidebarLeft.xhtml"/>is expecting to find that page in http://[SERVER]/[APPLICATION]/sections/login/sidebarLeft.xhtml instead of WEB-INF/flows/sections/login/sidebarLeft.xhtml
in any case, the object ExternalContext is being used (javax.faces.FacesException: java.io.FileNotFoundException: /sections/login/header.xhtml Not Found in ExternalContext as a Resource) to look for the resource ( http://download.oracle.com/docs/cd/E17802_01/j2ee/javaee/javaserverfaces/2.0/docs/api/javax/faces/context/ExternalContext.html ). Maybe some of these methods are being used:
URLfor the application resource mapped to the specified path, if it exists; otherwise, return
In this case the path parameter is like if was an context-based path value.
I would make some example pages in order to testing with the browser if the path is visible or not...