Error message 'please hold on for just a little bit more'

When I go to the theme>customise>Upfront
I keep getting the message 'please hold on for just a little bit more' but nothing happens, no matter how long I wait. And I've tried it on different browsers.

If I've understood correctly there should be an Upfront column for clicking/dragging/editing etc that should appear on the left and that doesn't happen. Nothing on the page can be clicked or edited.