there, i've installed custom sidebar plugin, went

hi there,
i've installed custom sidebar plugin, went to widgets, managed to create a new sidebar but when i click on 'Sidebar Location' it gives me the popup but it loads the content all the time and doesn't do anything. it seems to get stuck on loading it. please see the screenshot attached. eventually after a while it says: Couldn't load data from WordPress...
i've tried deactivating all the other plugins and using a default twentythirteen theme. also, i've installed another wordpress installation and tried that, but still the same result. the site is in the maintenance mode at the moment, but i could send you login details to have a look on it if that's possible, unless there's something obvious you think i could try doing?

thanks,
Adam