[fptex] problem with metapost and format.mp

[jacques-marot-10-13]

Metapost has a mysterious behavior for me.
This simple minimum program that I have made
to understand  where is my problem,  works
very well or not  at all according to the folder
 where is stored.

%begin mp  file
verbatimtex
\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}
etex

input format;
defaultfont:="ptmri7t";
beginfig(1)
     draw (0,0)--(10.5cm,0);
     for i:=0 step cm until 10cm :
         draw (i,-mm)--(i,mm);
         %label.bot( format("%g",decimal(i/(2cm))) ,(i,0));
     endfor;
endfig;
end.
%end mp file


When the unit format.mp is called, there is this error whose I join the
log-file below.

%begin log-file
This is MetaPost, Version 0.641 (Web2c 7.5.2) (mem=mpost 2004.6.7)  7 JUN
2004 20:09
(c:/Program Files/TeXLive/texmf/web2c/cp8bit.tcx)
**F:/DocTeX/personnel/test.mp
(f:/DocTeX/personnel/test.mp
(c:/Program Files/TeXLive/texmf/metapost/base/format.mp
(c:/Program Files/TeXLive/texmf/metapost/base/string.mp)
(c:/Program Files/TeXLive/texmf/metapost/base/texnum.mp
>> c:/Program Files/TeXLive/texmf/metapost/base/texnum.mp
>> texnum.mpx
! Unable to make mpx file.
l.14 init_numbers(btex
                      $-$etex, btex$1$etex, btex${\times}10$etex,
The two files given above are one of your source files
and an auxiliary file I need to read to find out what your
btex..etex blocks mean. If you don't know why I had trouble,
try running it manually through MPto -tex, TeX, and DVItoMP.



%end log-file

I still note this error when the line :
label.bot( format("%g",decimal(i/(2cm))) ,(i,0));
is commented, the constitution of a  mpx  file seems started by the simple
call to  the format.mp unit.
However if I record the file in this other folder :

f:/DocTeX/personnel/metapost/test.mp

 the  mpx file is made  without any problem,
I join the log -file below :

This is MetaPost, Version 0.641 (Web2c 7.5.2) (mem=mpost 2004.6.7)  7 JUN
2004 20:22
(c:/Program Files/TeXLive/texmf/web2c/cp8bit.tcx)
**F:/DocTeX/personnel/metapost/test.mp
(f:/DocTeX/personnel/metapost/test.mp
(c:/Program Files/TeXLive/texmf/metapost/base/format.mp
(c:/Program Files/TeXLive/texmf/metapost/base/string.mp)
(c:/Program Files/TeXLive/texmf/metapost/base/texnum.mp))
(c:/Program Files/TeXLive/texmf/metapost/base/marith.mp
(c:/Program Files/TeXLive/texmf/metapost/base/string.mp)) [1] )
1 output file written: test.1


In which direction do have I to seek to correct this random error?

Thanks for your help

Jacques !