If it’s not a PlugIn or Theme problem this will help:
Enter the following 3 lines into your wp-config.php
define('COMPRESS_SCRIPTS', false);
define('CONCATENATE_SCRIPTS', false);
define('COMPRESS_CSS', false);
And everything is back to normal!
Maybe not all 3 are nessecary. I found the solution here. Thanks!
Have fun!
Florian