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Are you sure you have chmodded the cache/, templates_c/ and uploads/ folders to 777 and the mainfile.php file to 666?
Also, this seems like a server config issue, where readdir() is disabled. It seems to me (but I'm not a specialist by a long shot) that this function is a basic function for PHP, and should be active by default. You could also consult your hosting provider.
Herko