Hi Pinastro! I’m a newbie here but I do know that SimpleScripts installations tend to be junkie and end up with 404 errors. It’s not that hard to install by hand — truly. Just take the instructions to install WordPress-MU, then BuddyPress, then BBPress bit by bit and it’ll be fine.
(Are you on Bluehost? I am and so I did first install with SimpleScripts and got the same errors so backed up, dropped the database tables and started over again.)