cache not working correctly

Dear Codeigniter Team,

We have a problem with Codeigniter framework.

We have created Information System with Codeigniter 2.2.2 which each user can logged in with username and password. It's run under apache 2.4 web server and php 5.6. First the system worked correctly, but after 3 weeks it's work wrong. When user logged to the system by their username and password, they will see on their page, something like "Welcome, James Wagner", but after clicking some of menu like they will see the page another user something like "Welcome, Ann Crud". When one user press Ctrl+F5 he/she will see the own page but other user will see his/her page.

We are sure about our code, because it works in another framework.

I reinstall apache and php, but the problem didn't solved.

There are about 16000 users in our software and we should solve it as soon as possible.
I hope you will help us to solve it.

Thank you. If you need any other question, we will happy to help you.