codeigniter session library

[eluser]einsteinity[/eluser]
hi, i have a site build with CI.
the site is working fine in the beginning.
but today i got report from a user that he and some others cannot log in from Chrome.
but he can log in from Firefox.

i tried to investigate the issue but i can use the site normally without any issue.
until i create this topic, the user still cannot log in.
seems like this issue doesn't happen to all user.

do anybody here have idea what cause this?
is it the session class or something else?