PHP fatal error when hierarchical_select' ist installed

When we have https://www.drupal.org/project/hierarchical_select installed we get a blank page when creating a new menu item. Also when updating an existing one. The Apache error log shows a

PHP Fatal error:  Cannot create references to/from string offsets nor overloaded objects in /our_path/includes/common.inc on line 6703, referer: http://our_path/admin/structure/menu/manage/main-menu/add?menu_mlid=0&menu_title=Forum&menu_plid=0&menu_name=main-menu&menu_hidden=0&menu_weight=1&menu_link_path&destination=admin/structure/menu/manage/main-menu&msg_buffer

When we switch off hierarchical select, all works fine.