Conflict with SEO Suite extension
If you are using Improved Layered Navigation and have SEO suite by MageWorx installed, please open file app/code/local/MageWorx/SeoSuite/etc/config.xml and remove these
<layer_filter_item>MageWorx_SeoSuite_Model_Catalog_Layer_Filter_Item</layer_filter_item>
<layer_filter_attribute>MageWorx_SeoSuite_Model_Catalog_Layer_Filter_Attribute</layer_filter_attribute>
<layer_filter_category>MageWorx_SeoSuite_Model_Catalog_Layer_Filter_Category</layer_filter_category>
and these lines
<layer_state>MageWorx_SeoSuite_Block_Catalog_Layer_State</layer_state>
<product_list_toolbar>MageWorx_SeoSuite_Block_Catalog_Product_List_Toolbar</product_list_toolbar>
Then clear cache.
