There is an additional whitespace at the beginning of XML (Sitemap) file, which is causing an XML error as you can see in the below links.
Issue is present irrespective of which plug-in is used to generate XML Site map. Refer http://onlinebicycles.in/sitemap_index.xml (from yoast) and http://onlinebicycles.in/sitemap.xml (from another XML plug-in).
Discussion forum says that a theme file could be adding an additional whitespace.
Please help urgently because it is degrading our Google ranking.