The increment_id column need to have the additional
'filter_index'=>'main_table.increment_id',
'filter_index'=>'main_table.increment_id',
Copy file
app/code/core/Mage/Adminhtml/Block/Sales/Order/Grid.php
app/code/core/Mage/Adminhtml/Block/Sales/Order/Grid.php to app/code/local/Mage/Adminhtml/Block/Sales/Order/Grid.php
app/code/local/Mage/Adminhtml/Block/Sales/Order/Grid.php
Find function _prepareColumns()function _prepareColumns()
Change
$this->addColumn('real_order_id', array(
'header'=> Mage::helper('sales')->__('Order #'),
'width' => '80px',
'type' => 'text',
'index' => 'increment_id',
));
To
$this->addColumn('real_order_id', array(
'header'=> Mage::helper('sales')->__('Order #'),
'width' => '80px',
'type' => 'text',
'index' => 'increment_id',
'filter_index'=>'main_table.increment_id',
));
