I found out that the Zend optimiser I had installed was causing the crashes in combination with the horde framework (php5 & apache2). After I uninstalled it, it ran fine!
I tried to run this with apache2 & php5 on winxp, configured everything properly added an imap server.... but apache crashed when I pressed the 'Folders' icon. And it kept on crashing on every horde framework page until session was lost. Further it seems to work fine though. I also think it's alot of stuff if you're just looking for webmail only.