My config:
IIS 6
PHP 5

With a particular configuration I have the following error :

SYSTEM WARNING: require_once(core.php) [function.require-once]: failed to open stream: No such file or directory

Indeed, in all pages of the graph plugin, I notice that the following line on the php page of the plugin is useless:
require_once( 'core.php' );

to have the plugin work properly , it is necessary to delete this line on all files (indeed, core.php is already included in plugin.php).

This update seems to work well in classic configuration (Apache/php)

I think the same error occurs on other plugins