I use CI for my website. And I have a login page. But in some browsers on some computers I can't login to the system, because no userdata is stored in browser. I can't figure out what is the difference between the (PC+Browser)s which can and can't login.
If you are using database for storing your CI sessions (table ci_sessions), make sure you change the default UserAgent field to VARCHAR 250 or something like that. Right now I think it's something rather small that doesn't accomodate the entire UserAgent. That way, I think the browser isn't identified
A second cause could be that you need to open config/config.php and edit this section