BuddyPress.org

>Buddypress is waiting for the first login of each user

At which point it will add a last_activity timestamp.
The easiest solution is to add those timestamps for everyone via a script.
Put this code in your bp-custom.php file, then open your site in a browser.
Then remove the code – or it will update the timestamp on every page load.

function daten_add_last_activity() {
    global $wpdb;
    foreach ( $wpdb->get_col( "SELECT ID FROM $wpdb->users" ) as $user_id ) {
        bp_update_user_last_activity( $user_id, bp_core_current_time() );
    }
}
add_action('bp_init', 'daten_add_last_activity' );