XF 1.5 Cache not working

[greenvoice]

Dear community, I have the following code:

if ($cacheObject = XenForo_Application::getCache())
{
    if ($cache = $cacheObject->load("MY_CACHE", true))
    {
        $myResult = unserialize($cache);
    }
    else
    {
        $myResult = $db->fetchAll("SOME QUERY");
        $cacheObject->save(serialize($myResult), "MY_CACHE", array(), 900);
    }
}

For some reason, `load` always returns empty string, despite `save` method is called proeprly and w/o erros. It seems like value in cache is lost.

I have enabled APC cache in my `config.php`.
Why this is happening?