Some days ago an l3kernel etc update was announced. It mentioned:

  - Move experimental l3luatex code out of l3kernel: this
    requires more work before inclusion in the 'stable' release
    bundle

But where has the code been moved too? Unicode-math actually did use
some of the code and now breaks due to the missing \lua_now_x:n:

\documentclass{article}
\usepackage{amsmath}
\usepackage{unicode-math}
\begin{document}
blub
\end{document}

! Undefined control sequence.
\__lltxmath_set_mathchar:NN ...def
#1\lua_now_x:n 

{lualatex.math.print_class...



(Adding 
\usepackage{expl3}
\ExplSyntaxOn
\let\lua_now_x:n\directlua
\ExplSyntaxOff
before unicode-math avoids the error)


-- 
Ulrike Fischer 
http://www.troubleshooting-tex.de/