Thanks for your great work too :)

Hi, I think I find the cause.

It works well on this site... There must be something strange on my own install...

Here is a screen record.

I'm at version a9e2e6c5aab94c8f4c57797ac63bfd88955a630d and php-fpm restarted after upgrading.
I don't have any editing permission on this install obviously, but the reproduce steps are the same. I see a 404 Not Found error on page after clicking "Edit Menu" rather than anything else.