I assume you are using the current v2.0?

order by m.manufacturers_name order by p.products_sort_order, pd.products_name limit 0, 15]
The problem is there are two order clauses.

This is on the product listing?

What do you have defined here?
\includes\languages\english\extra_definitions\product_listing_sorter_en.php

PHP Code:
define('PRODUCT_LISTING_SORTER_LIST''
      Name (asc)d.products_name;
      Name (desc)d.products_name DESC;
      Model (asc).products_model;
      Model (desc).products_model DESC;
      Price (low to high).products_price_sorter;
      Price (high to low).products_price_sorter DESC;
      Manufacturer:m.manufacturers_name;
      Newest.products_date_added DESC;
      Most Popular: p.products_ordered DESC
      '
);