You can not enable (publish) or disable (un-publish) a "default" menu item from "Menu Item Manager" list view; However, you can publish or un-publish via "Menu Item: [ Edit ]" on a single item.
If you unpublish from single item edit, you will get the same error message as below if you are trying to disable from menu manager list no matter the item already been un-published.
If we are no allow to disable "default" menu item, we should make it consistent or vice versa.You cannot unpublish the default menu item
I also noticed there are two notices on the top (debug mode on)
Notice: Trying to get property of non-object in /usr/local/apache2/htdocs/j15/includes/router.php on line 119
Notice: Trying to get property of non-object in /usr/local/apache2/htdocs/j15/includes/router.php on line 122