Can't adjust the design/header

Hi,

since the last update I can’t adjust/customize the design anymore.

When I try to activate the menu Design/Anpassen (german version) there is this URL http://die-kulturquelle.de/wp-admin/customize.php?return=%2Fwp-admin%2Fthemes.php%3Fpage%3Dsydney-info.php

and a get a white screen.

The problem is with Design/adjust, /menu and /header.
Any idea what to do?
Yvonne