At present, it is not possible to rename "system-defined" categories. Therefore, the "Rename Category" menu item in the Tools drop-down menu (gear icon) in the template manager (ctrl+shift+N) should be greyed-out (so that it is not possible to attempt to rename).
This is done now and the behaviour is a bit different in the various VCL backends. Gtk3 shows the rename item as greyed out, but kf5 does not show it at all.