Wolfgang,

Below is what I think is the bare minimum to explain this question
(sorry for not doing it right from the beginning...)

As I said, with the code as it is below, the header is not repeated
after the first page, but the table is in the right place.

Replacing "split=yes" by "split=repeat" makes the header repeat
correctly but puts the beginning of the table on the next page, leaving
the first page nearly completely blank.

In both cases, the "header=repeat" setting has no influence at all on
the behaviour and can be omitted without changing the results.

As I read in some other posts, sometimes putting the table in a float
can help, so I tried this (by removing the 3 "%" in the code below), but
there seems to be a conflict with the figure in the page header, and it
stops with an error message.

However, replacing the figure in the page header by a normal text
suddenly solves the problem: putting "split" in the setupfloat and
"header=repeat" in the setupxtable gives me a table with repeated
headers that starts exactly where I want it to start. Apparently in this
case, the "headers=repeat" is necessary. Unfortunately, I need a company
logo up in the page header, which is what causes the error for which I
have no explanation.

btw: I am using context standalone version: 2020.01.30 14:13

=====================================================

\setuppapersize[A4, landscape]

\setuplayout[location=middle, width=27.5cm, height=18cm,
backspace=1cm,header=4cm]

\setupcaptions[location=none]

\setupbackgrounds[header][text][background={Logos}, state=repeat]

\defineoverlay

[Logos][{

\framed[width=\textwidth, height=3cm, align=right, strut=no, offset=none]{

\framed[width=0.280\textwidth,height=3cm,align=right,]

{\placefigure[force][]{none}{\externalfigure[somepic.png][height=1.9cm]} }

\framed[width=0.430\textwidth,height=3cm,align=middle] {sometext in the
middle}

\framed[width=0.270\textwidth, height=3cm, align=middle]{sometext on the
right}

}

}]

%\setupfloat[table][default={force,split}]

\setupxtable[ option=stretch,split=repeat,header=repeat,align=middle]

\setupxtable[head][background=color,backgroundcolor=gray,foregroundcolor=red]

\starttext

Some lines of text. This text must come just before the table, but only
on the first page of the table

\def\onerow{

\startxrow

\startxcell first \stopxcell

\startxcell second \stopxcell

\startxcell third \stopxcell

\startxcell fourth \stopxcell

\startxcell fifth \stopxcell

\startxcell sixth \stopxcell

\stopxrow}

%\startplacetable

\startxtable

\startxtablehead[head]

\onerow

\stopxtablehead

\startxtablebody

\dorecurse{40}{\onerow}

\stopxtablebody

\stopxtable

%\stopplacetable

\stoptext