Why does uploading images fail in this application?

I am working on a Social Network application with Codeigniter 3, Ioan-Auth and Bootstrap 4. More details HERE.

The code below correctly hashes the filename before inserting it in the avatar column, but the upload itself never happens.

Code:

if ($this->form_validation->run() === TRUE)
    {
        //more code here

        $config['upload_path'] = './assets/img/avatars';
        $config['file_ext_tolower']     = TRUE;
        $config['allowed_types']        = 'gif|jpg|jpeg|png';
        $config['max_size']             = 1024;
        $config['max_width']            = 1024;
        $config['max_height']           = 1024;
        $config['encrypt_name']         = TRUE;
        $this->load->library('upload', $config);

        if (!$this->upload->do_upload('userfile')){
            $error = array('error' => $this->upload->display_errors());
            $file_name = null;
        } else {
            $file_name = $this->upload->data('file_name');
        }

        $additional_data = [
            'first_name' => $this->input->post('first_name'),
            'last_name' => $this->input->post('last_name'),
            'avatar' =>  $file_name,
            'company' => $this->input->post('company'),
            'phone' => $this->input->post('phone'),
        ];
}

I have a XAMPP server on Windows 10, so it is unlikely that there are permission issues on the images folder.