OMG, I'm an idiot! I've been working on putting in the publication dates, but when I went to test the new cover, I've CONSISTENTLY picked the next one to be worked on... in other words, a script with no publication date. So, it makes complete sense that it would show nothing.
The error disappeared after I restarted Calibre (I'd done two plugin updates and not bothered to restart as I wasn't working with those plugins).