Using a programmer's text editor, please make this edit. In /classes/catalogue.class.php, near line 893:
From:
$GLOBALS['catalogue']->searchCatalogue((int)$_GET['cat_id'], $page, $catalogue_products_per_page);
To:
$GLOBALS['catalogue']->searchCatalogue($_GET['cat_id'], $page, $catalogue_products_per_page);
The variable $_GET['cat_id'] could actually contain the value 'sale'. Should this text value be cast to a numerical type, it equals 0, for which there is nothing to be found.
The use of (int) was added in an attempt to solve some other problem.