WordPress SEO 1.4.22 Redirect 301 not working

On my Website http://www.pixelportal.de i made a url change in my backend from http://www.pixelportal.de/portfolio/ to http://www.pixelportal.de/referenzen/ i used the Advanced tab and under 301 redirect i placed the url http://www.pixelportal.de/referenzen/ but it does not work if I deactivate the plugin it works and opens the new url referenzen after activate it i get a blank page i need some advice.

http://wordpress.org/plugins/wordpress-seo/