[Plugin: WP Super Cache] Preload button stopped working

I noticed this evening that the preload button has stopped working. This is with the latest versions of WP and WP Super Cache. Clicking on Preload Now puts the Scheduled preloading of cache in 10 seconds message at the top of the page (it seems it used to be at the bottom), the Cancel Preloading never appears and nothing is ever preloaded.

I’ve tried uninstalling and reinstalling Suport cache, but not luck. Any suggestions?