[Plugin: WordPress SEO by Yoast] Portfolio page doesn't work after installing plugin

Hello,

I’m creating a portfolio page for my website, and it worked good.
I’m using this theme: http://themeforest.net/item/showtime-business-and-portfolio-wordpress-theme/117367

When I install this SEO plugin, the portfolio page-content is gone, everything on the site stay there, except the portfolio content, when I go to the direct URL, it works, only this page won’t work: http://demo.freshface.net/file/st/wp/?cat=5 (this is the theme page from themeforest, in my self-hosted site the content is gone, but the header, footer etc. remains.)

When I go to the direct URL: http://demo.freshface.net/file/st/wp/?portfolio=water-is-reaching-for-its-predecessors, it works.

Has anyone here got a solution for me?
Thanks! (sorry for my bad English….)

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