You can turn on debug mode in your rsobj.php file and it will give you the full error. The issue was that you had code:
$rsCLIENTNAME->setQuery("SELECT * FROM customers_address_book, customers WHERE customers_address_book.id = $cont AND customers_address_book.customer_id = customers.customers_id");
But your variable $cont is set on the top of the page with:
$cont = $_POST['contact_name'];
The submit of this page doesn't include the 'contact_name' form element, so once the page resubmits the code fails. The solution might be to just add a hidden form element named "contact_name" set to the previously submitted value so that it carries through to the next post.