[ESS] ESS M-x R failing in a Windows installation

[Steve Gutreuter]

M-x R is returning the message "Searching for program: No such file or 
directory, Rterm" under MS Windows.

R 4.0.3 is installed in c:\Program Files\R\4.0.3, and Rterm.exe is 
present in c:/Program Files/R/R-4.0.3/bin/x64.
My path environment variable includes c:\Program Files\R\4.0.3.
R starts fine outside of Emacs.
My .emacs file includes:
    (require 'ess-rutils)
    (setq inferior-ess-r-program "c:/Program 
Files/R/R-4.0.3/bin/x64/Rterm.exe)
I am using Vincent Goulet's emacs-26.1-modified-2.exe and 
emacs-27.1-modified-1.exe which bundle ESS within the Emacs installation.
Everything worked fine under Emacs 26.1 and earlier.  I upgraded to 
Emacs 27.1 and M-x R still worked.  I backed down to Emacs 26.1 and now 
M-x R is broken.

I am baffled as to why ESS is not finding R.  Any clues?

Many thanks
-- 
Steve Gutreuter