Session expired in design > adjust

(Foreword: I have a german wordpress-version and I am really sorry that I maybe do not use the exact english terms!!)

My problem: since I moved my wordpress installation and all the data to a new host I cannot use “design > adjust” anymore. I always get logged out. It says the session has been expired and the login screen is displayed where my blog-preview should be – with the respective adjustments-menu on the left. If I try to login nothing changes, the login screen is still displayed.
I tried other themes and other browsers, it won’t work.

Does anyone have an idea what could help?