Cannot add sidebar

I am using a theme from themeforest. It includes a custom sidebar creator. In the past I’ve never had issues creating a new sidebar. This week, however, anytime I try to add a sidebar it says “Ajax Error. Cannot Add Sidebar.”

I’ve tried disabling all plugins that have been recently added with no luck. No idea what to do here…this came out of nowhere.

I did update wordpress to 3.5.1 on Monday, which is also when the sidebar issue started. Don’t know if that could be related…

I’ve contacted the theme author’s support but his responsiveness has left much to be desired recently, so I’m trying this route.