It was a total new installation on php 5.3.1 with the IP 1.0.2 beta, no extra's done at all. Except that I did do the .htaccess trick which I posted earlier 'install in subfolder'. viewtopic.php?f=7&t=68&p=254#p254
I thought I did first check if it was related to this and as far as I remember it didn't. So maybe it's related to php 5.3.1? It looks like it only initiaties the first time a session is started and not after. But I'm really not sure as it felt intermittend!
Let me know if you want me to test something