Well, you define less levels than you need.
Try this :
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\definesectionlevels
[default]
[
%chapter, % uncomment this line to make it work
section, subsection, subsubsection, subsubsubsection, subsubsubsubsection]
\starttext
\startsectionlevel
\startsectionlevel
\startsectionlevel
\startsectionlevel
\startsectionlevel
\startsectionlevel
\startsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stoptext
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Interestingly, this will also work, although (if I’m counting correctly) there are still only 5 levels defined, but 6 are used (just like above):
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\definesectionlevels
[default]
[
%chapter, % uncomment this line to make it work
section, subsection, subsubsection, subsubsubsection, subsubsubsubsection]
\starttext
\startsectionlevel
\startsectionlevel
\startsectionlevel
\startsectionlevel
\startsectionlevel
\startsectionlevel
%\startsectionlevel
%\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stopsectionlevel
\stoptext
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Von: Sylvain Hubert