I just spent couple of hours by finding out, why is my Magento administration logging me out after moving my Magento store to new server. I found many hacks, modification and setup tips on the Google, but anything helped to me.This caused also dysfunction of the part of the administration, like image upload or switching among different parts of the Magento administration. Solution in my case was quiet easy, but hard to find:
I am using Suhosin php extension and I had to change one of the config values from:
suhosin.session.encrypt = On
suhosin.session.encrypt = Off
This solved my whole problem with sessions. Hope it helps to you too.