The old Google Custom Search Engine (CSE) site control-panel URL is now consistently returning ‘404 not found’, though the custom search-engines that sit beneath it are still working fine. There’s no automatic re-direct, just a plain ‘404 broken’ message.
The solution, if you have a browser bookmark that starts like this…
https://cse.google.com/cse/setup/
… is that you should now access your CSE control-panel(s) via…
https://programmablesearchengine.google.com/controlpanel/all
The latter is working fine.