Url rewrite problem/Mystery redirects

Hi,

I´m running a zalle store on my site, but I think, I´m sure, this problem is gerenal. My Wpmu Installation redirects Urls like:
http://www.mypage.com?gridPage=1_4&gridSort=&keywords

To http://www.mypage.com.

But why? The .htaccess is okay, the server support url rewrite, averything shoould be okay, but It does not work. I have absolutely no Idea, what the reason could be. Can anybode hep me, please?
You can reproduce this error on this page:
http://www.pc-store.org

Lars