duplich

By the way the case happened on the Multisite (path-based network)
3.4.1
and I got the following error om my apache:

[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP Catchable fatal error:  Object of class stdClass could not be converted to string in /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-includes/general-template.php on line 2274, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP Stack trace:, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   1. {main}() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-admin/customize.php:0, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   2. WP_Customize_Section->maybe_render() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-admin/customize.php:110, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   3. WP_Customize_Section->render() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-includes/class-wp-customize-section.php:72, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   4. WP_Customize_Control->maybe_render() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-includes/class-wp-customize-section.php:88, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   5. WP_Customize_Control->render() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-includes/class-wp-customize-control.php:130, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   6. WP_Customize_Control->render_content() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-includes/class-wp-customize-control.php:143, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   7. selected() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-includes/class-wp-customize-control.php:211, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren
[Mon Jul 23 14:45:14 2012] [error] [client 212.34.187.99] PHP   8. __checked_selected_helper() /var/www/wordpress/dev.wordpress.themes.mysite.de/wp-includes/general-template.php:2240, referer: https://dev.wordpress.themes.mysite.de/demo-makler/wp-admin/customize.php?save=Speichern+%26+Aktivieren

After trying the same Theme in the root site of my Multisite the preview was running smoothly…
Really weird.

I have noticed another problem. I also have a multisite instalation but the problem is in a theme.
I live preview functions without functions.php so I searched the problem there and found out that when I comment out the register_nav_menus() in the Theme preview is functioning again…
Maybe not relevant but the Theme has wp_nav_menu() in sidebar – it is a rework of a Twenty-ten org theme…
Anyone had the same problem with a theme not being supported?

Putting following code in your css:

#YourMenuId ul li ul{ display:none;}

Shoult be pretty helpful…