Hi Rene,
I am sorry for the late reply.
I have finally found Windows machine with IE9 and I can replicate this issue. However, after a further test on your site, I found out that this issue is coming from your template. When I changed to another template, the editor is displayed correctly. Here is my screenshot:
http://screencast.com/t/RZib3OHRbX . Can you consult with your hosting provider regarding this issue?
Looking forward to hear from you again. Please do update us back.
Please advise.
Thanks.