--- class-wp-xmlrpc-server.php	2014-01-23 07:49:36 +0200
+++ class-wp-xmlrpc-server.php.fixed	2014-01-23 07:44:43 +0200
@@ -359,7 +359,7 @@
 			'post_thumbnail'    => array(
 				'desc'          => __('Post Thumbnail'),
 				'readonly'      => true,
-				'value'         => current_theme_supports( 'post-thumbnails' )
+				'value'         => strval( current_theme_supports( 'post-thumbnails' ) )
 			),
 
 			// Updatable options