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