Note that changing the title of the item from "The Rhone from Sea to Source" (i.e. losing the accent on the o" doesn't make a difference. However, if I use that title to create the item I can save it as often as I want.

I can not confirm this issue. I followed the steps and could save the menu item several times without any warnings.

@prh47bridge What’s your backend language? And is the PHP extensions “intl” for internationalization (e.g. transliteration) enabled?

On further investigation, it only happens if the "modify it" step involves changing the parent item. Other changes to the menu item don't cause the problem.

My backend language is English (United Kingdom).

php-intl was not installed. I have now installed and enabled it but it has made no difference.

Just to add the specifics in case it is relevant, I created the menu item and forgot to assign a parent item. I then went in again to edit it and assign the correct parent. It refused to save. I deleted the menu item (trash and empty trash) and then tried again, assigning it to the correct parent when I created it. That worked. I can reliably reproduce the issue on my system.

I can't replicate it either - sorry

It is more specific than I thought! It seems to depend on which parent item I choose. I can move the menu item to some parents without a problem, others produce this error. I will add some debugging to see if I can figure out what's going on.

are you 10000% certain that you dont have any trashed menu items?

Oops! For some reason I didn't find the trashed item when I searched for it. It does exist. My bad. Sorry.