By doing a second install using Normal level security, then comparing webapp settings, I have tracked down the cause of my problem.

My install was a Minimal security one, after which I changed the settings of the /csp/sys web app and its sub-apps /csp/sys/* by clearing the Unauthenticated checkbox and setting the Password one.

But it seems that unless I do the same for the /api/interop-editors webapp I get a Rule Editor authentication page which I am unable to get past, not even by entering the credentials of the same %All-role user I logged into /csp/sys (i.e. Portal) with.

As soon as I made the change to /api/interop-editors I was able to get into the new Rule Editor, seeing only a brief flash of its authentication paged.

Here are the tweaked Minimal-security settings that work for me (i.e. authentication required to access Portal, from where I can move without challenge into the new Rule Editor):