Well, I got the error happening again this morning so I put some of the suggestions here to the test.

First, basicwiz's suggestion to use the "Render" menu item instead of using the buttons or "Render in background" did work. Partially! It rendered a new scene, overwriting the previously frozen-in-place scene, but it didn't stop the error happening on subsequent background renders. It just froze on the new scene I'd rendered using menu-item "Render".

Second, Vestmann's suggestion to switch to the Render tab before submitting a background render worked, too. The frozen-in-place render disappeared and the new scene began to be rendered as hoped for. Again, though, it didn't unlock this interesting feature but it does mean that I now know of two ways to work around it.

Third, lesbentley's "External binary morphs" suggestion isn't the problem. I learned in my P6 days to switch this evil off (just how many scenes did I lose back then until I'd discovered to do that?).

 

Thanks to everyone who replied.