Had tried to remove everything and then i did do a clean beta2 installation. But still it didn't work.
So i tried to clean up once more. And remove all sitefinity files I could find on my computer.
Now it works again...
Don't know what went wrong, the only difference from 1st try and 2nd was that the 2nd time I deleted everything in %SystemRoot%\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET folder.
Now it works and thats the main thing :-)
Thanks for your quick reply