WP Smush Pro Not Working, Response, success false

WP Smush Pro isn't working.

In Chrome developer tools, Network, I can see the requests go, but the responses are like this:
{"success":false,"data":{"size_before":39303423,"size_after":35347727,"percent":10.06,"human":"3.77 MB","bytes":3955696,"smushed":56,"total":133}}