There could be several causes, but first and foremost make sure you have created a primary taxonomy item in each minisite. If you have not created this then using some parts of the site will cause a parser error. Obviously you don't have to worry about that if you don't have any minisites.
Secondly, make sure you have uploaded/installed the theme correctly. You should not have uploaded Flavor-Theme (if using Flavor) or Steam-Theme (if using Steam) because then your AJAX file will be in the wrong location. In other words, the location of your theme's style.css file on your server should be here:
.../wp-content/themes/flavor/style.css (or /steam/ if using Steam)
.../wp-content/themes/Flavor-Theme/flavor/style.css (or /Steam-Theme/steam/ if using Steam)
If you uploaded it the wrong way and don't want to risk moving it on your server, you can open the functions/ajax.php file and edit line 7:
$wordpress_location = "../../../../wp-load.php";<br>
Add one more up level like this:
$wordpress_location = "../../../../../wp-load.php";<br>
If that still doesn't fix the issue, disable all of your plugins and see if it begins working. A known plugin that conflicts for instance is http://wordpress.org/plugins/shorten-url/
Basically all of the theme's ajax calls are getting blocked because your web host has mod_security turned on. You need to follow the steps in that article to get it working. Your web host will be able to work with you on fixing this. This is very important because the theme passes a URL as a variable to the ajax file, and your web server is set up to block that. It is important to note this is not a theme issue but rather an issue specific to how your web server is setup.
Lastly, see this ticket with a further solution if none of this fixes the issue for you: