I've just noticed that my current version of KMail (5.4.0, latest openSUSE Tumbleweed packages 16.12.0-2.1) doesn't allow me to change folder names any more (via folder properties). It just seems to ignore any changes I enter after clicking ok.

This is most likely an Akonadi problem, because the same happens also in the browser of Akonadi Console.

Is there anything obvious I'm missing?

I'm more than happy to provide further information/testing.

Please file a bug report. This looks like a regression of some sort.

Thanks you for the reply; I've filed a bug report.