Why does tabularx fail with siunitx?

Question

Some time ago I wrote a document using tabularx and siunitx together. It compiled well in TL2016, but now on TL2017 (pretest) it gives me an error about

(c:/texlive/2017/texmf-dist/tex/latex/siunitx/siunitx-abbreviations.cfg)
! Missing } inserted.
<inserted text> 
                }
l.14        \end{tabularx}

? 

MWE (attention: you have to uncomment one line to see the error):

\documentclass{scrreprt}
\usepackage{array}
\usepackage{tabularx}
\usepackage[locale=DE]{siunitx}

\begin{document}
    \begin{table}[h]
        \caption{Vergleichende Darstellung der Pro-Kopf-Kosten in €}
% uncomment the following line to see the error
%       \renewcommand{\tabularxcolumn}[1]{S[table-column-width=#1,table-figures-decimal=2]}
        \begin{tabularx}{\linewidth}{@{}lXXXX@{}}
            \textbf{Position} & \multicolumn{2}{c}{\textbf{Criterion 1}} & \multicolumn{2}{c}{\textbf{Criterion 2}}\\
            1 & 220,00 & 0,00 & 0,00 & 220,00\\
            2 & 0,00 & 1,80 & 0,00 & 1,80
        \end{tabularx}
        \renewcommand{\tabularxcolumn}[1]{p{#1}}
    \end{table}
\end{document}

My question is especially what has changed and causes this problem and how to solve it for TL17.

Btw: I do know about How to use siunitx and tabularx together? and that solution does currently work, but my question is specifically about what causes this conflict now.

Update: File List

 *File List*
scrreprt.cls    2017/04/13 v3.23 KOMA-Script document class (report)
scrkbase.sty    2017/04/13 v3.23 KOMA-Script package (KOMA-Script-dependent basi
cs and keyval usage)
 scrbase.sty    2017/04/13 v3.23 KOMA-Script package (KOMA-Script-independent ba
sics and keyval usage)
  keyval.sty    2014/10/28 v1.15 key=value parser (DPC)
scrlfile.sty    2017/04/13 v3.23 KOMA-Script package (loading files)
tocbasic.sty    2017/04/13 v3.23 KOMA-Script package (handling toc-files)
scrsize11pt.clo    2017/04/13 v3.23 KOMA-Script font size class option (11pt)
typearea.sty    2017/04/13 v3.23 KOMA-Script package (type area)
   array.sty    2016/10/06 v2.4d Tabular extension package (FMi)
tabularx.sty    2016/02/03 v2.11 `tabularx' package (DPC)
 siunitx.sty    2017/03/23 v2.7f A comprehensive (SI) units package
   expl3.sty    2017/05/13 L3 programming layer (loader) 
expl3-code.tex    2017/05/13 L3 programming layer 
l3pdfmode.def    2017/03/18 v L3 Experimental driver: PDF mode
  xparse.sty    2017/05/13 L3 Experimental document command parser
 amstext.sty    2000/06/29 v2.01 AMS text
  amsgen.sty    1999/11/30 v2.0 generic functions
l3keys2e.sty    2017/05/13 LaTeX2e option processing using LaTeX3 keys
translator.sty    2010/06/12 ver 1.10
translator-language-mappings.tex
translator-basic-dictionary-English.dict    
siunitx-abbreviations.cfg    2017/03/23 v2.7f siunitx: Abbreviated units
 ***********

Answer 1

This is due to a change in expl3 interacting with some low-level code in siunitx: the latter is there due to a slight 'gap' in the coverage of useful functions by expl3. For the present, adding

\ExplSyntaxOn
\cs_set_protected:Npn \hbox_set:Nw  #1
  { \tex_setbox:D #1 \tex_hbox:D \c_group_begin_token }
\cs_set_eq:NN \hbox_set_end: \c_group_end_token
\ExplSyntaxOff

will fix the issue. I will update one or both of expl3 and siunitx today to address this long-term. (The above is a sticking-plaster solution for the immediate issue, but not the best overall solution in the long run.)