I'm trying to add a new user through the admin panel, but when I click Save I get the error message "Sorry, this user is protected from being altered in the config.php file by the $config['SpecialUsers']['undeletableusers'] variable." I checked config.php but that field was empty, just as I thought. Any ideas what could cause this?

Go ahead and post that exactly line in thats in your config.php

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// ****** UNDELETABLE / UNALTERABLE USERS ******
// The users specified here will not be deletable or alterable from the control panel by any users.
// To specify more than one user, separate userids with commas.
$config['SpecialUsers']['undeletableusers'] = '';

Try putting your userid number between the ' '. That way you are telling the forum who is the undeletable member is.