Unable to load the "Craft\xyz" runtime after Craft update
Fixed by changing:

class xyz extends \Twig_Extension
{
    //...
    public function getFunctions()
    {
        return array(
            new \Twig_SimpleFunction('kex_cache_breaker', array(__CLASS__, 'cacheBreaker'))
        );
    }
    //...
}


To:

class xyz extends \Twig_Extension
{
    //...
    public function getFunctions()
    {
        return array(
            new \Twig_SimpleFunction('kex_cache_breaker', array($this, 'cacheBreaker'))
        );
    }
    //...
}