categories don’t work

Hi there,

I just installed your plug-in and I absolutely love it so far, but there is one problem I can’t figure out. I use the Sydney wordpress theme and I was just creating the portfolio menu by displaying the categories as a dropdown under the portfolio. But now when I go to my website and click on one of the categories (shown in as a dropdown by the portfolio tab) my website says ‘oops, this page couldn’t be found’ (but than in dutch because I’m from the Netherlands). Can you help me solve this problem? Thank you.

The page I need help with: [log in to see the link]