Wordpress - Forbidden, White pages, broken content

Hi guys,

I am new on IIS ;)

I installed the latest WordPress Version (via Web Plattform Installer) on my IIS 8 server. After a few days some strange problems showed up.

- When I save a page, post, plugin or option - I get a white blank page, but the content is saved and published. Or I get "Forbidden".

- I can`t upload an image to a post, it also shows me a empty media library - but in the media library itself, I see all images

- Some JS and CSS files are filled with the content "NULL"

Whats wrong here? Can you please help me?

Thanks