I got the following error in the minimal example (which is all shown in the
output below)

system          > tex > error on line 4 in file simpl.tex: Undefined
control sequence ...

1     \setuphead[section][before={\column}]
2     \starttext
3     \startcolumns[n=2]
4 >>  \section{this is a section}
5     \section{another section in a new column}
6     \stopcolumns
7     \stoptext
8


\208>multicolumn:yes ->\par {\testrulewidth
                                            \zeropoint \ruledvskip
\textheig...
\page_breaks_columns_process ...dle_direct \v!yes
                                                  \fi \relax \the
\everyafte...
<to be read again>
                   \relax
\dohandleheadpagebreakyes ...ter \c!before \relax
                                                  \doif {\headparameter
\c!a...
\docheckheadbefore ...se \continuoussectionhead #1
                                                  \fi
\doheadspacingbeforeyes ...handleheadpagebreakyes
                                                  \headparameter
\c!inbetwee...
...
l.4 \section{this is a section}

Basically, the first line in the file introduces the error.  It worked a
few weeks ago.