Error "POST" 500 with ajax request.

I've created a modal register form and when i submit the form i get the following error:

Code:

jquery-3.3.1.min.js:2
POST http://localhost/[website_name]/users/create_user 500 (Internal Server Error)

I've checked the controller and model and i cant find any errors.
Any ideas?!

Controller

Code:

 public function create_user(){
   $email = trim( $this->input->post('email') );
   $email_exist = $this->user_model->check_email_exist($email);

   if ($email_exist) {
     $result['status'] = 'failed';
     $result['message'] = $this->lang->line('error_email_exist');
   } else {
     //Pass enc
     $enc_password = md5($this->input->post('passInput'));
     $register = $this->user_model->register($enc_password);
     if ($register) {
       $result['status'] = 'success';
     } else {
       $result['status'] = 'failed';
       $result['message'] = 'Please fill all the fields';
     }
   }
   return json_encode($result);
 }

Model

Code:

 public function register($enc_password){
   $user_name = convert_accented_characters($this->input->post('screenNameInput'));
   $user_name_slug = url_title($user_name);

   $data = array();

   $data['user_group_id'] = $this->input->post('userGroupInput');
   $data['screen_name'] = $this->input->post('screenNameInput');
   $data['username'] = $this->input->post('usernameInput');
   $data['email'] = $this->input->post('email');
   $data['password'] = $enc_password;
   $data['slug'] = $user_name_slug;
   $data['register_status'] = 1;
   $data['unique_id'] = random_string('numeric', 6);

   if ( $this->input->post('userGroupInput') == 1 && $this->input->post('websiteInput') ) {
     $data['website'] = $this->input->post('websiteInput');
   }

   $this->db->insert('users', $data);
   $user_id = $this->db->insert_id();

   $contact_data = array(
     'primary_num' => $this->input->post('phoneInput'),
     'info_group_name' => $this->input->post('screenNameInput'),
     'city' => $this->input->post('cityInput'),
     'to_user_id' => $user_id
   );

   $registered = $this->db->insert('contact_info', $contact_data);

   if ($registered) {
     return true;
   } else {
     return false;
   }
 }

Ajax function:

Code:

   $('#userRegister').on('submit', function(e){
     e.preventDefault();
     var form = $(this);

     $.ajax({
       url: form.attr('action'),
       type: "POST",
       data: form.serialize(),
       dataType: "json",
       success: function(data){
         if(data.status == 'success') {
           window.location.href = 'users';
         } else if (data.status == 'failed' ) {
           $('.error-messages').html(data.message);
           $('.error-messages').fadeIn('fast');

           setTimeout(function(){
             $('.error-messages').fadeOut('fast');
           }, 2000);
         }
       }
     });
   });

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