Native session PHP error with codeigniter 2.2

Hi people, i have a problem with the native sessions on php with codeigniter.
I have a ecommerce, and i use a native session because I think it is safer, also because the sessions of codeigniter cannot be larger than 4k.
But i have a problem, sometimes the sessions work correctly, that is to say, the items are added to cart, etc, but but at other times, items not are added to cart. And then I have to close the browser and reopen.
A someone else happens this error ?

i use this library to extend library session: https://github.com/atomicon/codeigniter-native-session

how I can fix this problem, I have no idea what can cause the error.
Sorry for my poor english