Thanks you Bernard an John for your comments.
I created a new simple 260 item menu just unique name and the same url.
This works as expected when editing, deleting an item or adding a new item.
I created a new cloned menu id 42 in my live web site, also on a fresh V24.2 install new database, and also on a V 25.0Alpha install new database. In all cases i get a "leave site? dialog when trying to save an edit, delete or create new item.
When i try to edit my current menu created some time ago using Export of menu 42 clone and edit the .csv and then import, i get the 403 error previously mentioned.
I can't believe that i am the only one with this problem over the last 3 years or so, unless i am doing something wrong.... but what?