on the products detail, it looks like the ID for the WA_Store_Cart_1_ID_Add form element has been removed.
Find:
<input type="hidden" name="WA_Store_Cart_1_ID_Add" value="<?php echo $row_WADAProducts["ProductID"]; ?>" />
and change it back to:
<input type="hidden" name="WA_Store_Cart_1_ID_Add" id="WA_Store_Cart_1_ID_Add" value="<?php echo $row_WADAProducts["ProductID"]; ?>" />