- creating that directory, with the same owner and group as settings.php
This is probably your problem. For a directory to be considered a directory, it MUST have execute permissions. Common configurations are 775 or -drwxrwxr--, depending on how your control panel visualizes it.
the directory has permissions 755, owned by the www server user; isn't it enough? Also: must it be in the drupal installation root or in (for virtual hosting) drupalroot/sites/virtualdomain_1/files ??
D.