cookie issue, can you help me?

hello
sorry for my english, i have an unknow issue on my website:

i try to save a cart in a cookie:

var_dump of the cookie before saving :

Code:

array (size=3)
'name' => string 'cart'
'value' => string '[{"id":"13991","id_prod":"18176","qty":1,"gravure":"","options_infos":[],"cadeau":0},{"id":"13983","id_prod":"18168","qty":1,"gravure":"","options_infos":[],"cadeau":0},{"id":"13987","id_prod":"18172","qty":1,"gravure":"","options_infos":[],"cadeau":0},{"id":"279543","id_prod":"29990","qty":1,"gravure":"","options_infos":[],"cadeau":0},{"id":"279554","id_prod":"30001","qty":1,"gravure":"","options_infos":[],"cadeau":0},{"id":"279539","id_prod":"29986","qty":1,"grav ure":"","options_infos":[],"cadeau":0},{"id'... (length=3301)
'expire' => int 2592000

this php function is triggerered on my page to update the cookie cart,
this is what is blocking when i have more than approx  20 items on my cart

PHP Code:

public function refreshCookie() {
$items = array();
if (parent::contents()) {
foreach (parent::contents() as $key => $value) {

$prod = new stdClass();

$prod->id = $value['id'];
$prod->id_prod = $value['id_prod'];
$prod->qty = $value['qty'];

if (isset($value['gravure'])){
$prod->gravure = $value['gravure'];
} 
if (isset($value['options_infos'])){
$prod->options_infos = $value['options_infos'];
}
$prod->cadeau = $value['cadeau'];
$items[] = $prod;
}
}

$cookie = array(
'name' => 'cart',
'value' => json_encode($items),
'expire' => $this->CI->config->item('cookie_panier_lifetime')
);
$this->CI->input->set_cookie($cookie);
} 


this work fine in my local machine but not online when i have more than 20 items
it wotk if i remove the set_cookier line ($this->CI->input->set_cookie($cookie);)
also note that if i use the php setcookie it is the same, so it is not realy a codeigniter issue :/


i don't understand , any help would be appreciated
regards