Looks like that is a problem with the Memcached server itself, not Q2A or PHP. First check that your Memcahced server is running, which you can do with a plain PHP script like this:
<?php
$m = new Memcached;
$m->addServer('127.0.0.1', 11211);
print_r($m->getStats());
If you Google that error there are many resources with solutions, not sure which ones will work for you.