When logged in phpbb 3.0 logs members out automaticly.
At the logs you can see a error log like this:

Session IP/Browser check failed
»User IP "22.134.." checked against session IP "26.242.." and user browser string "" checked against session browser string "".

It happens to everyone who is logged in.
I`m using apache 2.2.2
PHP Version 5.1.4

I hope everything is clear, i`m dutch so i`m not that good at my English, sorry for that!