Good afternoon,

the recent version change of expl3 from [2017/04/01] (last TL-16 state) to
[2017/05/13] (initial TL-17 state) changed the behaviour of \hbox_s:Nn, which
made a hot-fix necessary for one of my packages (ocgx2) on CTAN.

The problem boils down to the following code, which used to work with expl3 as
of v. 2017/04/01, but fails now with `! Missing } inserted.' error.

    \hbox_set:Nn0\bgroup CTAN    % broken since expl3 [2017/05/13]
  %  \setbox0=\hbox\bgroup CTAN  % hot-fix

I would like to return to using \hbox_set:Nn instead of \setbox.