Layout Builder not working

Hi there,
I’m using the Sydney theme on WP 4.4.2. I’ve just tried to add a row (that was OK) and then add a Layout Builder widget (that was OK too). But when I click to edit the widget I get a white box with an Open Builder button. Strange, but clicking on it does nothing.

It’s worked in the past when I’ve tried to use it and there is nothing different to my site other than the recent update of WP.

Any ideas? Thank you in advance.