Uninstall, reboot and then reinstall 1.43 beta.
My OP said that I ran the Start11 purge script, which I would tend to think would have accomplished the same thing as uninstalling and then some, but I have done the uninstall/reboot and now there's a slightly different problem where the keyboard Start button will alternate between the Start11 menu and the standard Windows menu, momentarily show the Start11 menu and then open the standard Windows menu, or else layer the Windows menu over the Start11 menu at the same time.
[EDIT] I re-deleted the Stardock\Start8 configuration branch from the registry and let it recreate itself. Start11 is at least largely usable again, but still some wonkiness remaining. I've been messing around with things a bit and I'm noticing that I get different results depending on what is in context when I click the KB Start button. If certain objects are in focus, such as a file dialog or registry editor, I get the Windows menu. If other objects are in focus, I get the Start11 menu.