Dear Fabrice,

The following code is working in my mac. I tried your first code and it is working after adding \setupinteraction[].
Since I didn¡¯t use ¡®rsteps¡¯ module, I could¡¯t test the second code.

My ConTeXt version is as following.

ConTeXt  ver: 2014.12.21 22:25 MKIV beta  fmt: 2014.12.26  int: english/english


\setupinteraction[state=start]
\setupcolors[state=start] 

\usemodule[pre-60]

\setuppapersize[S4][S4]

\starttext

\StartSteps 
\startformula \startalign
\NC x^2 + y^2  \NC  = z^2 \NR \FlushStep
\NC   x^2 + y^2   \NC = z^2 \NR  \FlushStep
\stopalign\stopformula
\StopSteps
\stoptext

I hope that this helps.

Best regards,

Dalyoung