I hit this strange bug while trying out the animation layout in Krita 4.3. When I'm working in the default animation workspace any layer I create can't be renamed. The good news is I just have to change the workspace to something else and make the changes. Afterwards I can just change back to the animation workspace.

BernieBn, is it possibly because you're trying to change the name in the Timeline docker instead of Layers docker? Timeline docker has much less options for layers than the Layers docker.