CodeIgniter 3.1.2内部服务器图像上载错误

I am getting :

500 Internal server error 

on Image upload through AJAX. When it reaches:

 $this->upload->do_upload('filename');

it produce this error. I search every where but no solution found.

My code is:

foreach($_FILES as $key_img=>$row_img){
    if(!empty($row_img['name'])){
        $image_path = './assets/images/';
        $config['upload_path'] = $image_path;
        $config['allowed_types'] = "gif|jpg|png";
        $config['max_size'] = '1000';
        $this->load->library('upload', $config);
        if (!$this->upload->do_upload($key_img))
        {
            $upload_error = array('error' => $this->upload->display_errors());
        }
        else
        {
            $data = array('upload_data' => $this->upload->data());
            $userfile = $data['upload_data']['file_name'];
        }

    }
}

The error reported in Console where as no response on Network Tab.

There is a problem with the codeigniter 3.1.2 upload.php, I have php7 but I get the 500 error. My solution was to download codeigniter 3.0.6 and copy the file upload.php and paste it into system / libraries