Can't save feed

I’ve tried creating an initial feed source, but the animated icon just keeps spinning when I click “Save Feed”. I did the same thing with a different rss plugin, and everything worked fine right away. What could be the problem? I know that the feed URL is fine; the url works fine in a browser.

In the wp_posts table, I can see that the plugin creates an initial post with the new feed’s name, but it is in draft form. After that, nothing happens. This is in WP 4.0.