Please see my post here:- https://www.zen-cart.com/showthread....22#post1324122

In the admin/customers.php file - around lines 402 - 404 there is an incorrect field label for the TABLE_PRODUCTS_NOTIFICATIONS value.

Code:
$db->Execute("delete from " . TABLE_PRODUCTS_NOTIFICATIONS . " where customers_id = " . $customer_id);
Should be...

Code:
$db->Execute("delete from " . TABLE_PRODUCTS_NOTIFICATIONS . " where customers_id = " . $customers_id);
The line does not exist in 1.5.5b .