You should upgrade to v1.3.7 as there are many issues in v1.3.5 that have been fixed ...

Look in phpMyAdmin at the products table ...

Do you see valid categories_id in the master_categories_id

The master_categories_id needs to match the categories_id that is on sale or the category on sale does not own the product and the price cannot work right ...

Also be sure you do not have products and categories mixed in the same product ...

Have you an URL where this issue is happening?