No Longer Regenerates, Stuck on Processing

Saw a similar thread from a few days back and so I checked the console, as suggested there. Showing an error getting “domain.com/wp-json/rta/regenerate” from the server. Clicking the regenerate button on the settings page gives that error, and the network tab shows it fetches it twice when that button is pressed; first one goes through, second returns HTTP 500.

When I check the REST route directly, I get this: ” {“code”:”rest_no_route”,”message”:”No route was found matching the URL and request method”,”data”:{“status”:404}}”. For the life of me, I can’t see a setting or even a line of text in the plugin documentation that says I need to install WP REST plugin.

Have gotten the above results on two of three sites tested; the third had the REST API v2 plugin installed. Trying to regenerate a single thumb from the media page has the same problem. I’ve also had the same problem with versions 1.5.1.1 and 1.5.2. Running WP 4.8.1 (one of the three installs is new).

Any ideas? Thanks!