Just digged a little deeper into aps_crawler.inc.php:
In the function public function startCrawler() it's able to fill the $apps_to_dl array with the correct information about Files.
But after this part of the function:
// For memory reasons, unset the current vendor and his apps
it seems to stop doing anything. Strange. memorylimit is 512MB, so this should be sufficient. How can I trace the problem further???