gistfile1.php

$user_creds = array(
	'user_login'		=> $member_data['member-username'],
	'user_password'		=> $member_data['member-password'],
	//'remember'		=> false
);

$user = wp_signon( $user_creds, true );
if( is_wp_error( $user ) ) {
	$result['redirect'] = site_url( '/login/?error=failed_login' );
}
else {
	file_put_contents( ABSPATH . '/log.txt', print_r( $user, true ) );
	//wp_set_current_user( $user->ID, $user->data->user_login );
	wp_set_auth_cookie( $user->ID, false, true );
	//do_action( 'wp_login', $user->data->user_login );
	$result['redirect'] = site_url( '/join/become-a-member/get-started?id=' . $user->ID );
}

Try to hook in set_auth_cookie and check, if params are set. If everything is good there then setcookie is failing for some reason.