Don't worry, Holland is still fine...


But the problem I refer to is not in the dimension whether or not it is spelled correcltly.
The error is still the same:

! Undefined control sequence.
<argument> \reporttextprefixerror

\secondoftwoarguments #1#2->#2

\rightlabeltext ...ts \let \reporttextprefixerror
                                                 \doreporttextprefixerror \...
\labeltexts ...eftlabeltext {#1}#2\rightlabeltext
                                                 {#1}
\doifundefinedelse #1->\ifcsname #1
                                  \endcsname \@EA \secondoftwoarguments \el...
<argument> ... {\??ef \??ef XY\currentheadnumber }
                                                  {\useexternalfigure [\s!d...
...

l.14 ...er][width=\the\dimexpr.5\textwidth\relax]}


\starttext
\chapter{One}
\placefigure
       [here]
       []
       {A figure}
       {\externalfigure[XY\currentheadnumber][width=\the\dimexpr.5\textwidth\relax]}
\stoptext

Willi