If you face problems with WordPress after having updated the 3.5 version you can try the following.

In the wp-config.php file, right before this line:
require_once( ABSPATH .'wp-settings.php' );

add the following line:
define( 'CONCATENATE_SCRIPTS', false );

Once version 3.5.1 will become available you can try to remove the CONCATENATE_SCRIPTS line and see if the problem is definitely gone.

If you found a different solution please share it with a comment!