In addition to the change Sue is suggesting for the config.php file, you will want to add:
$conf['cookie_prefix'] = "";
And I would then verify that 666 are the correct permissions on your host for PHP to be able to write to the file, by going to your General Configuration and changing the “URL to Documentation Directory” preference, making sure that it “sticks” when you change it.
If you find that value above in your config.php file already, and its version was already correct, then the update went okay, the file was written to properly, but a PHP caching mechanism is probably still holding the old information. I’ve seen that occur on MAMP environments, for instance.