Minimal example:

\definepapersize[PaperSize][
  width=400mm,
  height=100mm,
]

\setuppapersize[PaperSize]

\starttext
  \startcolumnset[n=3]
    \dorecurse{3}{\input knuth}
  \stopcolumnset
\stoptext

The old-columnsets module fails with an error. The following works as expected:

\definepapersize[PaperSize][
  width=400mm,
  height=100mm,
]

\setuppapersize[PaperSize]

\starttext
  \startcolumns[n=3]
    \dorecurse{3}{\input knuth}
  \stopcolumns
\stoptext