Unable to add menu items

I am creating a new website, using the Vantage theme. I was unable to automatically update to WordPress 4.4.1, and instead did it using filezilla. But although it said it installed successfully, my ‘add to menu’ button is not working.

When I try to add items to the menu, nothing happens – the little icon just keeps on circling to show that it’s ‘busy’.

According to suggestions on previous posts about this issue, I should check whether it’s the theme or plugin causing the problem. But I’ve tried it on the 2015 theme, as well as disabling my plugins, and the problem persists – I still cannot add menu items.

Not sure whether this is a problem within the theme or a problem with my WP installation itself. Any advice please?

Website: https://www.geoffreywright.co.za