[Xenon]

you must have this block in your user.php:

PHP Code:

    if ($exists=$DB_site->query_first("SELECT userid
                    FROM user
                    WHERE username='".addslashes(htmlspecialchars($ausername))."'
                        AND userid <> $userid")) {
        echo "There is already an ".makelinkcode('user',"user.php?do=edit&userid=$exists[userid]",1)." named <b>".htmlspecialchars($ausername)."</b>";
        exit;
    } 


if it doesn't exist in this form you don't have uploaded an original file.

as you can see the block is just executed if there is another user called "....".
so if you have this block correct, and and get this error when editing a user, you perhaps have a problem in your database.