Hi Coolio,
I have checked and seen the cause is SQL error because you enable Flat catalog category settings from admin. To solve this issue, please go to the file: \app\code\core\Mage\Catalog\Model\Resource\Category\Flat.php, find the code 'main_table.custom_design_apply', and replace by: // 'main_table.custom_design_apply',.
Thanks you,
