We had a user report an issue and were able to duplicate it. When Smush Pro (v. 2.8.0) is enabled on a site, we cannot add an image to the slider using the Spacious theme (v. 1.5.2) . Spacious has a built in slider. The Choose Image button is disabled. When checking the Console in Chrome, we get several errors including: "Failed to load resource: the server responded with a status of 500 (Internal Server Error) for /site/wp-json/wp/v2/:1. Disabling the WPSmush Pro plugin resolves the issue.
I have a screenshot if that will help. Just not sure how to add it here.