I've got the same problem with http://mundschenk.at. When the "Use CDN" feature is enabled, the URL used by the plugin gives a 404.
I've disabled the feature (to use the local copy of the script) and visitors are tracked again in the Spy View, but not the Right Now tab. A manual call to wp-slimstat-js.php gives the expected "-105 : invalid data format" error message.