[Plugin: NextGEN Gallery] thumb problem

I have this warning when i upload images.
what is?

Warning: require_once(/web/htdocs/www.myaddresse.it/home/test2/wp-content/plugins/nextgen-gallery/admin/ajax.php) [function.require-once]: failed to open stream: No such file or directory in /web/htdocs/www.myaddresse.it/home/test2/wp-content/plugins/nextgen-gallery/nggallery.php on line 234

Fatal error: require_once() [function.require]: Failed opening required '/web/htdocs/www.myaddresse.it/home/test2/wp-content/plugins/nextgen-gallery/admin/ajax.php' (include_path='.:/php5/lib/php/') in /web/htdocs/www.myaddresse.it/home/test2/wp-content/plugins/nextgen-gallery/nggallery.php on line 234