I have fixed the plugin by editing the following file:-
google-places/googleplaces_settings_html.php
Find the following lines of code and ‘comment’ them out.
// $geo_sitemap_xml[]='<geo:geo>';
// $geo_sitemap_xml[]='<geo:format>kml</geo:format>';
// $geo_sitemap_xml[]='</geo:geo>';
Support for the ‘geo’ tag has been removed by Google, so it’s no longer required.