I just installed this plugin, and sure enough, if I try to post without a category, the little popup box appears... but when I click OK, the little "hourglass" circle keeps spinning, and the "Publish" button stays dark and unclickable. Checking categories does not affect this.
I'm using the Graphene theme, and about a dozen other plugins.
Any thoughts? This is pretty much the lowest priority of my plugins; if the answer is "plugin conflict", I'll just uninstall this one. But if there's a better answer, I'd like to know...