This bug is a direct result of bug #819585. If you choose the wrong session when logging in the first time, the session is set wrong and causes the Xubuntu session to retain the wrong settings. More specifically,
XDG_CONFIG_DIRS end up containing /etc/xdg/xdg-default/ instead of /etc/xdg/xdg-xubuntu
This bug is a direct result of bug #819585. If you choose the wrong session when logging in the first time, the session is set wrong and causes the Xubuntu session to retain the wrong settings. More specifically, xdg-default/ instead of /etc/xdg/ xdg-xubuntu
XDG_CONFIG_DIRS end up containing /etc/xdg/