i have the same problem it not serious but it annoying. after i run around i found my opend_basedir is on and have correct path in it. then i fix it by modify the core plugin
by remove “/” in includes/WPSupersized.php line 1515 it will look like this
if(is_dir($directory_base_path.$filename."/"))
and the annoying is gone i dont know it have effect to plugin or not but now it work fine for me
