The following example fails with recent betas, but not with TL18:

\setupinteraction [state=start]
\setuptagging     [state=start]
\setupbackend     [export=yes,xhtml=yes]
\setupexport      [title={Title}]
\setupstructure   [state=start]
\starttext
\startchapter[title={Chapter title}]
\startparagraph
  Chapter introduction.
\stopparagraph
\startsection[title={Section 1 title}]
\startparagraph
  Section one text.
\stopparagraph
\stopsection
\startsection[title={Section 2 title}]
\startparagraph
  Section two text.
\stopparagraph
\stopsection
\startsection[title={Section 3 title}]
\startparagraph
  Section three text.
\stopparagraph
\stopsection
\startsection[title={Section 4 title}]
\startparagraph
  Section four text.
\stopparagraph
\stopsection
\startsection[title={Section 5 title}]
\startparagraph
  Section five text.
\stopparagraph
\stopsection
\startsection[title={Section 6 title}]
\startparagraph
  Section six text.
\stopparagraph
\stopsection
\startsection[title={Section 7 title}]
\startparagraph
  Aaaa aaaaa aa aaaaaaa (aa aaaaaa) aaaaaaaa, aaa aa aaa aaaa aaaa aaaa aaaa aaa aaaaaaaaaa aa aaaaaaaa aaaa aaaa aa aa aaa aaaaaaaa. Aa aaaa aaaaaaaaaaa aaaaa, aaaa aaaaa aaaaaaaaaaaaaa aa aaaaaaaaa, aa aaaaa, aa aaaaaa aaa aaaa-aaaaaa. aaa’a aaaa aa aaaa aaa aa—aaaaa aaaaaa aaaa, aa aaa aaaaa. Aaaa aaaaa aa aaaaaaa aaaaaaaa, aaa aa aaa aaaa aaaa aaaa aaaa aaa aaaaaaaaaa aa aaaaaaaa aaaa aaaa aa aa aaa aaaaaaaa. Aaaa aaaaa aa aaaaaaa aaaaaaaa, aaa aa aaa aaaa aaaa aaaa aaaa aaa aaaaaaaaaa aa aaaaaaaa aaaa aaaa aa aa aaa aaaaaaaa.
\stopparagraph
\stopsection
\startsection[title={Section 8 title}]
\startparagraph
  Aaaa aaaaa aa aaaaaaa (aa aaaaaa) aaaaaaaa, aaa aa aaa aaaa aaaa aaaa aaaa aaa aaaaaaaaaa aa aaaaaaaa aaaa aaaa aa aa aaa aaaaaaaa. Aa aaaa aaaaaaaaaaa aaaaa, aaaa aaaaa aaaaaaaaaaaaaa aa aaaaaaaaa, aa aaaaa, aa aaaaaa aaa aaaa-aaaaaa. aaa’a aaaa aa aaaa aaa aa—aaaaa aaaaaa aaaa, aa aaa aaaaa aaaaaaaaaa aaaaaaa.
\stopparagraph
\stopsection
\stopchapter
\stoptext

--
Rik