Hans,

Thanks for your quick reply.  I adapted the file, according to your 
suggestion. The letter now comes out right! Thank you for that. I 
immediately tried an items list, which also works!

Still, if there is any \ code in the xml file, it does not seem to work.
Changing \"e to ë in Drie\"eenheid makes the ë disappear.

<contact ...>
...
<p>Laan der Drie\"eenheid</p>
...
</contact>

comes out as:

...
Laan der Drie“”eenheid
...

I also found out that \usemodule[m-letter] works much better than 
\usemodule[letter], which is not evident from the way is Context set up.

Thanks again,

Robert