[ESS] (Aquamacs) Why is text at R prompt read-only, cannot delete?

[Christian]

Hi,
Starting Aquamacs 3.3 on MacbookPro MacOS 10.12.6, I get an error
--------
Warning (initialization): An error occurred while loading 
‘/Users/hoffmannc/.emacs’:

File error: Cannot open load file, No such file or directory, 
~/elisp/vendor/ess/lisp/ess-site.el
-------

My /Users/hoffmannc/.emacs looks like this
_________________
;; ESS: Emacs Speaks Statistics
(load "~/elisp/vendor/ess/lisp/ess-site.el")

;; Use shift-enter to split window & launch R (if not running), execute 
highlighted
;; region (if R running & area highlighted), or execute current line
;; (and move to next line, skipping comments). Nice.
;; See http://www.emacswiki.org/emacs/EmacsSpeaksStatistics,
;; FelipeCsaszar. Adapted to split vertically instead of
;; horizontally.

(setq ess-ask-for-ess-directory nil)
   (setq ess-local-process-name "R")
   (setq ansi-color-for-comint-mode 'filter)
   (setq comint-prompt-read-only t)
   (setq comint-scroll-to-bottom-on-input t)
   (setq comint-scroll-to-bottom-on-output t)
   (setq comint-move-point-for-output t)
   (defun my-ess-start-R ()
     (interactive)
     (if (not (member "*R*" (mapcar (function buffer-name) (buffer-list))))
       (progn
     (delete-other-windows)
     (setq w1 (selected-window))
     (setq w1name (buffer-name))
     (setq w2 (split-window w1 nil t))
     (R)
     (set-window-buffer w2 "*R*")
     (set-window-buffer w1 w1name))))
   (defun my-ess-eval ()
     (interactive)
     (my-ess-start-R)
     (if (and transient-mark-mode mark-active)
     (call-interactively 'ess-eval-region)
       (call-interactively 'ess-eval-line-and-step)))
   (add-hook 'ess-mode-hook
         '(lambda()
            (local-set-key [(shift return)] 'my-ess-eval)))
   (add-hook 'inferior-ess-mode-hook
         '(lambda()
            (local-set-key [C-up] 
'comint-previous-matching-input-from-input)
            (local-set-key [C-down] 'comint-next-input)))
   (require 'ess-site)
_________________
..and the last line seems to cause the error, as can be seen by C-x C-e.
How can I correct this error?

Thanks for any hints.
Christian
-- 
Christian Hoffmann
Rigiblickstrasse 15b
CH-8915 Hausen am Albis
Switzerland
Telefon +41-(0)44-7640853