Hi There,
Thank you for the credentials but I was not able to see the error. But base on the error you provided above its the plugin ce-wp-menu-per-page. I was able to open the “Zuhause” page in Cornerstone so it seems deactivating the ce-wp-menu-per-page plugin works
But the Cornerstone very slow to load so I added this lines to your wp-config.php file.
define( 'WP_MEMORY_LIMIT', '512M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
Unfortunately, it did not improve the loading time as it seems your site/hosting plan is limited to only 128M. Also try deactivating this plugin (iThemes Security) and see if that improves the loading time of Cornerstone.
Another thing, please optimize this images and other images (these images are too big in filesize) from your site as it hinders the loading time.
http://g-culture.com/wp-content/uploads/Kickoff-Meeting-Messe-Frankfurt2-2017-03-23-for-web.jpg
http://g-culture.com/wp-content/uploads/Kickoff-Meeting-Messe-Frankfurt-2017-03-23-for-web.jpg
Thanks,