Attribute doesn't work in seo mode

It can be caused by Fishpig_AttributeSplash extension.

To solve it go to 

/app/code/community/Fishpig/AttributeSplash/etc/config.xml 

Find 

<layer_filter_item>Fishpig_AttributeSplash_Model_Layer_Filter_Item</layer_filter_item>

And replace it by

<!--<layer_filter_item>Fishpig_AttributeSplash_Model_Layer_Filter_Item</layer_filter_item>-->

Clean magento cache and re-compile.

Now it should work.