bbPress.org

Looks like that comes from bb-settings.php (line 156 in 0.9.0.1):

// Die if no URI

if ( !$bb->uri && ( !defined('BB_INSTALLING') || !BB_INSTALLING ) ) {

bb_die( __('Could not determine site URI') );

}

I think that says “If there is no $bb->uri defined AND you’re not installing, then die with this message.”

So, in your bb-config.php, what do you have defined for $bb->uri?