I’m the maintainer of the nodetree package. I got a bug report: https://github.com/Josef-Friedrich/nodetree/issues/46
I was able to narrow down the problem:

debug.tex:

\documentclass{article}
\begin{document}
\directlua{
  luatexbase.add_to_callback('vpack_filter',
    function(head)
      if node.type(head.id) == 'whatsit' and head.subtype == 1 then
        local tmp = head.data
      end
      return true
    end,
    'debug'
  )
}
\begin{figure}
  \caption{test}
\end{figure}
\end{document}

lualatex debug.tex:

! Incomplete \iffalse; all text was ignored after line 19.
<inserted text>
\fi
l.19 \end{figure}

The line 'local tmp = head.data' causes the error. I tried also 'print(head.data)' or 'local tmp = head['data']'. These access types also caused a crash.

Access to fields other than data works without any problems.

  print(head.stream)
  print(head.id)
  print(head.xxx)

Is this a bug in LuaTeX? Or how can I access the field 'data' of a whatsit(write) node for debugging purposes?