Empty popup box when attaching image to a post from media gallery

I’ve been getting an empty popup dialogue box when trying to attach an uploaded image to a particular post (for the [gallery] code to work).

When I check an image in the media gallery hit ‘attach’, a HTML dialogue titled “Find posts or pages” comes up with a search field, close button, and select button, but nothing else. Back a few versions of WP it used to show a list of the most recent posts which I could select from, but now nothing. When I type a word in the search field and hit ‘search’, a small spinner appears for a few seconds then disappears without showing anything.

It looks like it may be a display issue, but not relating to my browser or machine (have tried on both Safari and Firefox, and on two different Macs.

When I use Safari’s inspect element command, I think I can see the search results in the code, but the <div id="find-posts-response"> shows a height of 6px. I’m not a code guy, but would this be the issue? Here’s a screengrab showing what I’m seeing.

Is there a workaround to this? At the moment I cannot use the [gallery] shortcode in my post because the images aren’t attached to the post.