Only on the admin side. The front-end is fine.
Replacing the 'skin' folder did the trick!
So, to summarise:
Now admin login works. I suppose it makes sense to replace adminhtml with it's corresponding css files, doesn't it!
i have the same issue after updating my magento from 1.9.0.1 to 1.9.2.1 so i cant put products in the cart and the backend is broken. i tried to upload the missing css files and folders but i have still the same issue
Fatal error: Call to a member function addLink() on a non-object in /kunden/375433_12161/webseiten/happyfabric_08_2014_kopie/magento/HappyFabric_Shop_2014_MIT_WP/app/code/core/Mage/Adminhtml/Controller/Action.php on line 113