consider me being blonde: I just saw another upgrade being available for SlimStat, didn't care too much (at least I'm honest) and ended with the fopen-errors other users have reported before.
Made me take a closer look on your plugin page (ah, nice, the reason was written in bold), which led me to delete the plugin entirely and upload a fresh plugin file set into my plugins directory.
It seems to work then, but I get a nasty
Warning: array_flip() [function.array-flip]: Can only flip STRING and INTEGER values! in /var/www/bla/html/wp/wp-admin/includes/menu.php on line 194