This also occurs with WP 3.7.1 (noted below)
Using Yootheme Nano3 - their support properly identified this as a plug-in issue and not a theme issue.
Changing WP DEBUG in wp-config to TRUE I see the following on the Website:
Notice: Undefined property: stdClass::$type in /home/dev-site/public_html/wp-content/themes/dev-theme/warp/systems/wordpress/layouts/widgets.php on line 36
The "FOOTER" sidebar disappears (WP 3.8) from the Appearance > Widgets page and the Footer DIV disappears from the front-side of the Website.
The "FOOTER" sidebar is visible but cannot be accessed (WP 3.7.1) from the Appearance > Widgets page and the Footer DIV is gone from the front-side of the Website.
I troubleshooted by deactivating ALL plugins. After activating all but Adminimize I was able to access the Footer Sidebar as expected and replace the widgets. Once Adminimize was again activated the Footer sidebar disappeared from Appearance > Widgets (3.8) or was no longer accessible (3.7.1).
Nice plug-in but it is causing layout issues I cannot resolve.