Custom Sidebars pro: infinite loading

Hello,
I have a MU install running on my server and I just installed the Custom Sidebars pro plugin and activated it.
But now when I press on "Sidebar location" the popup opens but it loads infinitely and doesn't stop.
I tried network activate it, activate it only on one site, i installed it with the WPMU Dashboard and also downloaded the zip and installed it manually - unfortunately nothing worked.
I already created 9 sidebars before I installed the plugin - but I think thats not the problem, because on a new site that I created on the network I have the same problem when I create a new sidebar.
Could you tell me what the problem is? I use the "the7" theme.
thanks and greetings.