Thank you guys, you are the best!
I removed those media queries from the CSS + i had to remove a js file from the function.php file and it’s a lot better now. It’s far from perfect but it’s a progress and i’m really happy about it because this site gave me a headache since two days. I used Firebug to examine the CSS and i always missed those lines.
Right now the main div is the only one which still moves when i resize the browser window. That is the only thing i have to fix.
I can’t believe it. You saved my day. :) Cheers!