Session not work on server

[eluser]aherus[/eluser]
Hi guys,

i know that is an old topic, but i'm in developing an app for my client and still get the session issue on his server. On my localhost everything works perfect. I checked 10 times my code for whitespaces or closed ?> tag, tried 10^6 solutions over internet and nothing works for me. I really get nervous with this. It's not my first app in CI, but first that doesn't work properly. Maybe someone of You could help me solve the problem, I'll be very grateful.

Code:

A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /var/www/clients/client61/web237/web/medmar/system/application/controllers/admin_controller.php:1)

Filename: libraries/Session.php

Line Number: 675

I noticed that this error occurs when i tried load the session lib, via controller, model or autoload. Can it be a server config issue? mod_rewrite = on. Any solutions or inspiration?