Hi Gerben,
I puzzeled with your project code.
First you are not allowed to use \starttext \stoptext in a definition.
Your definitione interferes with the chapter mechnism. So more knowledge
is necessary which I do not have at the moment.
in the "before" option you say \page=right, seems to create
Fro my environment:
%\setuppapersize[B5][B4,landscape]
\setuppapersize[B5][B5]
\setupindenting[small]
\setuplayout[location=doublesided]
%\setuparranging[2SIDE]
\setuppagenumbering[alternative=doublesided]
%
\usemodule[lettri]
\def\ClearChapterQuote{\def\ChapterQuote{}}
\ClearChapterQuote
\newcomma