OK, yes, this is basically the same issue.
Sonoma for some (all, many, most, a few, I'm not sure) menus returns error -6520 menuNotFoundErr for menus it clearly finds. So selecting Safari File ➤ New Window, works, but then returns menuNotFoundErr. And trying to shoe Safari File ➤ Export fails with the same error.
So I would guess Apple has to work to do to resolve this. It's pretty repeatable, and the menu system is all new, so hopefully they will actually resolve it. Otherwise eventually I'll have to consider ignoring the error (if that is possible for all cases). But hopefully Apple will do the right thing and get this fixed.