Problems on login

[uwe]

Hello,

I have a strange problem, whenever i login, with correct user name and password (log table shows correct login), im always brought back to the login page, it returns 302 Found ! i.e:

POST index.php --->
302 admin/index.php <------
---->GET admin/index
302 index.php <---------
---->GET index.php
200 OK <-----------
This is an older build, RC4 (yes, yes, i will upgrade when i can) , march 2008! all users come from a single ip (gateway of campus lan).

This then works fine later ... for no clear reason :S, before breaking again, how can i debug it, where should i start ?