Ok. I've changed the undefs to '' and then to {}.
It now looks like this:

if (defined($in{new_aff}) && $in{new_aff} == 1) {
            if ($in{tax_id} ne "") {
                $dbh->do (qq{ UPDATE reg_users SET tax_id = ? WHERE us
+ername = ? },
                            {}, $in{tax_id}, $username);
            }
            $dbh->do (qq{ UPDATE reg_users SET aff_url = ? WHERE usern
+ame = ? },
                        {}, $in{aff_url}, $username);

            $dbh->do (qq{ UPDATE reg_users SET aff = ?, aff_su_date = 
+? WHERE username = ? },
                        {}, "yes", $formated_date, $username);
        }
        $dbh->do (qq{ UPDATE reg_users SET html_email = ? WHERE userna
+me = ? },
                    {}, $in{html_email}, $username);

        $dbh->disconnect();
[download]

Same Effect. It updates ONLY the columns: "aff" and "aff_su_date"!

Any other ideas?

Thx,
Richard.