Buddypress: Change Site-Wide Activity to Activity

Add it in functions.php in your theme:

add_filter( 'bp_get_directory_title', 'change_activity_title' );

function change_activity_title($data) {
 if( $data == 'Site-Wide Activity' ) {
  $data = 'Activity';
 }

 return $data;
}

Buddypress: 404 Errror Quick Fix

Re-saving the permalinks structure. Settings < Permalinks.