I updated the webassist/ckeditor/config.js file, i added:
config.filebrowserBrowseUrl = "webassist/kfm/index.php?uicolor=eee&theme=webassist_v2&showsidebar=true&startup_folder=cms_files";
if (window.cms_root != null) config.filebrowserBrowseUrl = cms_root + "/" + config.filebrowserBrowseUrl;
in the CKEDITOR.editorConfig function.
I think you may have created this CMS System with an older version of Design Extender, this is fixed in the most recent version.