Please try commenting out that block of code like this, and let me know if that fixes the problem:
/** if($acc_need_css == '1'){
define('S5_ACC_PATH', WP_PLUGIN_URL . '/mod_s5_accordion_menu/');
function s5_acc_internal_styles(){echo '<link rel="stylesheet" href="'.S5_ACC_PATH.'css/s5_accordion_menu.css" type="text/css" />';}
add_action('wp_head','s5_acc_internal_styles');} **/?>
Our demo site doesn't seem to have that problem, so it could be something specific to your server. If you notice, the error message is saying that it's trying to repeat that line a second time, which is somewhat unusual. If that fixes it, great. If not, I'll be glad to take another look and see if there's something else that can be done.