Actually, I recently added mailchimp extension & that is causing the problem with magento 1.9 is anyone aware of a solution?
The query doesn't understand from which table the 'created_at' field in WHERE clause should be considered. Find out the area and add the corresponding table alias to it, like main_table.created_at.
Probably the mailchimp extension has overridden the orders grid block.
EDIT: Override your Sales order Grid Block 'Mage_Adminhtml_Block_Sales_Order_Grid' and modify the 'created_at' column to the following
$this->addColumn('created_at', array( 'header' => Mage::helper('sales')->__('Purchased On'), 'index' => 'created_at', 'type' => 'datetime', 'width' => '100px', 'filter_index' => 'main_table.created_at' ));
Here we are adding a 'filter_index' to remove the ambiguous error.