I've just noticed that my current version of KMail (5.4.1, latest openSUSE Tumbleweed packages 16.12.1-1.1) 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.
5.4.2