Cache clearing fails in MODX3.0.2 – PHP

When I try to clear the cache from Manager or from a snippet on MODX3.0.2, MODX does not empty the folders under core/cache/resource/ and the cache won’t be cleared.

I traced the problem to xPDOFileCache.php:

public function delete($key, $options= array()) {
    $deleted= false;
    if ($this->getOption(xPDO::OPT_CACHE_MULTIPLE_OBJECT_DELETE, $options, false)) { 
        $cacheKey= $this->getCacheKey($key, array_merge($options, array('cache_ext' => '')));
        if (file_exists($cacheKey) && is_dir($cacheKey)) {
            $results = $this->xpdo->cacheManager->deleteTree($cacheKey, array_merge(array('deleteTop' => false, 'skipDirs' => false, 'extensions' => array('.cache.php')), $options));
            if ($results !== false) {
                $deleted = true;
            }
        }
    }
    $cacheKey= $this->getCacheKey($key, $options);
    if (file_exists($cacheKey)) {
        $deleted= @ unlink($cacheKey);
    }
    return $deleted;
}

If condition on line 3 returns false so deleteTree won’t be executed and the folder won’t be emptied. Could someone tell me, what is that condition about and why it returns false? Is it something related to my installation?