When I visit your sign in page the html at the top looks strange and the title of the page. I think you have html in your site title that may be causing some problems. You should not put html in the site title like <br /> etc, it should just be text. It gets html encoded so viewing the source on your sign in page I see:
Dottnet Klubben<br/><i style="padding-left:50px;font-size:50%;">"just playing.....</i> - Sign In
inside the<title>
In fact if you click around the pages that work and look at the title bar in the browser you will see problems. Home page doesn't show it but other pages do.
You should only enter plain text as the title. If you need html, it should go in layout.master, you can remove the site title control and put your own html there.
Hope it helps,
Joe