Metapost problem

Hi all,

Please consider a file called test.mp, whose contents are:

prologues := 0;
beginfig(1)
   dotlabel.top(btex $A$ etex,(0,0));
endfig;
end

I have MiKTeX (64-bits version) installed in three different machines. 
In one of them, if I run

mpost test.mp

all goes well. But in the other two, I get this error message:

This is MetaPost, version 1.999 (MiKTeX 2.9 64-bit) (MiKTeX 2.9 64-bit) 
  19 MAR 2016 18:56
**test
(C:/Program Files/MiKTeX 2.9/metapost/base/mpost.mp
(C:/Program Files/MiKTeX 2.9/metapost/base/plain.mp
Preloading the plain mem file, version 1.004) ) (./test.mp
 >> test.mp
 >> test.mpx
! ! Unable to read mpx file.
l.3   dotlabel.top(btex
                         $A$ etex,(0,0));
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 MPtoTeX, TeX, and DVItoMP

Why is it?

Best regards,

José Carlos Santos

------------------------------------------------------------------------------
Transform Data into Opportunity.
Accelerate data analysis in your applications with
Intel Data Analytics Acceleration Library.
Click to learn more.
http://pubads.g.doubleclick.net/gampad/clk?id=278785231&iu=/4140
_______________________________________________
Q: How can I leave the mailing list?
A: See http://docs.miktex.org/faq/support.html#leavingml