bbpress and simplepress plugin activation fails

Hi,
I've got the latest wordpress, multisite, buddypress and your social theme. And I am trying to get an integrated forum to work. But I've got a problem. Instantly after I activate the plugin (bbpress plugin or simplepress) the entire admin area either goes blank or gives a 500 internal server error :s
I've got no clue why this is happening.

What could the problem be you think?

Thanks
/Robert