Hans,

I'm getting this error if I put an image within a table using the export backend:
! LuaTeX error /usr/share/texmf/tex/context/base/back-exp.lua:297: bad argument #3 to 'format' (string expected, got boolean).

Minimal example:
---
\setupbackend[export=yes]
\starttext
% \externalfigure[mill] % -- fine
\bTABLE
% \bTR \bTD test \eTD \eTR % -- fine
\bTR \bTD \externalfigure[mill] \eTD \eTR % -- error
\eTABLE
\stoptext
---

Log attached.

Adam