Note, this is not a problem in MkIV, just LMTX

Error with 20211008 ConTeXt:

ConTeXt  ver: 2021.10.08 20:18 LMTX  fmt: 2021.10.9  int: english/english

...

tex error       > tex error on line 1 in file ./mmm.tex: Undefined control sequence \??markingfilter



<argument>
    \csname \??markingfilter
    \v!default \endcsname {chapter}
<macro> \sectionheadmarkingtitle
    #1#2->\clf_markingtitle {#1}{#2
    }
<macro> \strc_markings_get_yes
    ...rking \begincsname \??markingclass #1\endcsname {#2}{#3}}\fi \orelse \ifparameter #2\or \markingcommand {#1}{\csname \??markingfilter #2\endcsname {#1}}\else \markingcommand {#1}{\csname \??markingfilter \v!default \endcsname {#1}}
    \fi \endgroup ...
<to be read again>
    
    \fi
<macro> \page_layouts_process_element_indeed
    ...andcolor #1#2\begincsname \??layouttextstrut \layoutelementparameter \c!strut \endcsname \ignorecrlf \edef \p_width {\layoutelementparameter #3}\ifempty \p_width \else \expandafter \page_layouts_process_element_limited \fi \p_text
    \endgroup
<macro> \currentlayoutelement
    ...kern -\makeupwidth \fi \letfromlayoutelementparameter \p_text \c!righttext \ifempty \p_text \kern \makeupwidth \else \hbox to \makeupwidth \bgroup \hss \page_layouts_process_element_indeed \c!rightstyle \c!rightcolor \c!rightwidth
    \egroup \fi
 ...

<line 3.12>
    \stoptext

 1 >>  
 2     %\setupheadertexts [][][{\it\getmarking[chapter]}][]  % fails
 3     \setupheadertexts [][][{\getmarking[chapter]}][]     % fails
 4     %\setupheadertexts [][][\getmarking[chapter]][]       % succeeds

MWE to create. Note that the \it is not necessary to trigger the error, just the braces.

%\setupheadertexts [][][{\it\getmarking[chapter]}][]  % fails
\setupheadertexts [][][{\getmarking[chapter]}][]     % fails
%\setupheadertexts [][][\getmarking[chapter]][]       % succeeds

\starttext

\chapter{abc}
abc
\chapter{def}
def
\stoptext

--
Rik

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