Hello,
** Fabrice Couvreur [2018-08-23 10:48:42 +0200]:

> Hi,
> Thanks for your help but ...

I checked again and it is fine. May be I was too unspecific. First, I
changed a bit your example to this one:

[test.tex]
-------------------------------- 8< ------------------------------------
\setupsynctex[state=start]

\startusableMPgraphic{NumberHead}
  draw outlinetext.f
  ("\bf\namedheadnumber{chapter}")
  (withcolor "lightgray")
  ysized 50pt ;
\stopusableMPgraphic

\unexpanded\def\processMPheadnumber#1%
  {\useMPgraphic{NumberHead}}

\setuphead%
[chapter]%
[command=\HeadTitle,%
 headstyle=\ss,%
 numbercommand=\processMPheadnumber]

\unexpanded\def\HeadTitle#1#2%
{\framed%
  [frame=off,%
    bottomframe=on,%
    width=broad,
    align={broad,nothyphenated,left}]%
{#1\blank[nowhite]#2}}

\starttext
\startchapter[title=First chapter]
  \input ward
\stopchapter
\stoptext

%%% Local Variables:
%%% mode: context
%%% TeX-master: t
%%% End:
-------------------------------- 8< ------------------------------------

And second, I tweaked a bit my Emacs configuration:

-------------------------------- 8< ------------------------------------
(setenv "TEXMFCNF" nil)
(setenv "TEXROOT" "/usr/local/opt/context/tex")
(setenv "TEXMFCACHE" (format "%s/.cache" (getenv "HOME")))
-------------------------------- 8< ------------------------------------

(I have to do that because I have both TeX Live and ConTeXt
Standalone/Suite in parallel, and use mainly LaTeX.)

You should check your Emacs configuration.

This is my Emacs configuration:
https://bitbucket.org/vp1981/scripts/src/master/config/emacs/
and I have
Emacs:  27.0.50 (git)
AUCTEX: 12.1 (git)

[...]

---
WBR, Vladimir Lomov

-- 
Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me, from the crown to the toe, top-full
Of direst cruelty! make thick my blood,
Stop up the access and passage to remorse
That no compunctious visiting of nature
Shake my fell purpose, not keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature's mischief! Come, thick night,
And pall the in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark,
To cry `Hold, hold!'
		-- Lady MacBeth