Unable to add featured image to slides

I’m unable to add featured images in the “Add new slide” page. I click the “Set featured image” button, select an image, click “Set featured image” (or whatever the wording is in the English version), the pop-up disappears but the Slide Image box doesn’t update with the image I selected.

I’ve tried it in Firefox and Chrome on Mac. Latest version of the plugin, latest version of WordPress.

https://www.ads-software.com/extend/plugins/responsive-slider/