Package soul Error: Reconstruction failed in \tableofcontents and bibliography

Question

I'm writing a book in a scrreprt format. And, for the spacing and underlining the sections, I'm using the soul package. However, it throws an error in tableofcontents and bibliography. I'm clueless how to rectify this problem.

Here is the sample code:

...

\documentclass[a4paper, 12pt, headsepline, twoside, openright, parskip,fleqn]{scrreprt}

\usepackage{filecontents}
\begin{filecontents}{xyz.bib}
    @article{two,
        title={Die neuen Vierventil-Ottomotoren fur die mittlere Braureihe von Mercedes-Benz (Germany)},
        author={Abthoff, Von J and Huttebraucker, D and Zahn, W and Bockel, H},
        journal={Mtz},
        volume={53},
        pages={496--508},
        year={1992}
    }
    @inproceedings{three,
        title={Strategies for NOx reduction in heavy duty engines},
        author={Al-Sened, A and Karimi, E},
        booktitle={Proceedings of the 23rd CIMAC Congress},
        year={2001}
    }
    
    @misc{four,
        title={Deutsche Stiftung Weltbevölkerung},
        author={Bundesministerium für Wirtschaft und Arbeit},
        year={2003/04},
        publisher={NOAA}
    }
    
\end{filecontents}

\usepackage{soul}
\usepackage{soulutf8}
\usepackage{anyfontsize}
\usepackage{titlesec}
\titleformat{\chapter}
{\normalfont\fontsize{14}{16.8}\bfseries}
{\ul{\thechapter\enspace}}
{0em}
{\ul}
\setul{0.3pt}{1.5pt}
\titleformat{\section}
{\normalfont\fontsize{12}{15}\bfseries}
{\ul{\thesection\enspace}}
{0em}
{\ul}
\titleformat{\subsection}
{\normalfont\fontsize{12}{15}\bfseries}
{\ul{\thesubsection\enspace}}
{0em}
{\ul}
\titleformat{\subsubsection}
{\normalfont\fontsize{12}{15}\bfseries}
{\ul{\thesubsubsection\enspace}}
{0em}
{\ul}


\usepackage{fontspec}

\usepackage{unicode-math}
\setmainfont{Arial}
\setsansfont{Arial}

\setmathfont{GFSNeohellenicMath.otf}%equation font in arial font
\setmathfont[range=up]{Arial}
\setmathfont[range=it]{Arial Italic}
\setmathfont[range=bfup]{Arial Bold}
\setmathfont[range=bfit]{Arial Bold Italic}

\usepackage[numbers]{natbib}
\bibliographystyle{IEEEtranN} 
\bibpunct{/}{/}{;}{n}{}{,}
\renewcommand{\bibnumfmt}[1]{/#1/}
\begin{document}
\tableofcontents

\chapter{Thermodynamics}
\section{Heat}
Propagation of turbulent flames \cite{two} in spark-ignition engines is considered from the viewpoint of the different possible regimes \cite{three} of premixed turbulent combustion.\cite{four} Nondimensional parameters defining known combustion regimes are reviewed, 

    \medskip
    
    \bibliography{xyz}

\end{document}

...

Answer 1

It turns out that soul doesn't like \@mkboth at all.

If you don't need headers, the solution is to locally neutralize the command.

Beware that scrreprt doesn't really like titlesec. And also that underlining (especially so tight) is not typographycally very sound.

\begin{filecontents}{\jobname.bib}
    @article{two,
        title={Die neuen Vierventil-Ottomotoren fur die mittlere Braureihe von Mercedes-Benz (Germany)},
        author={Abthoff, Von J and Huttebraucker, D and Zahn, W and Bockel, H},
        journal={Mtz},
        volume={53},
        pages={496--508},
        year={1992}
    }
    @inproceedings{three,
        title={Strategies for NOx reduction in heavy duty engines},
        author={Al-Sened, A and Karimi, E},
        booktitle={Proceedings of the 23rd CIMAC Congress},
        year={2001}
    }
    
    @misc{four,
        title={Deutsche Stiftung Weltbevölkerung},
        author={Bundesministerium für Wirtschaft und Arbeit},
        year={2003/04},
        publisher={NOAA}
    }
    
\end{filecontents}

\documentclass[a4paper, 12pt, headsepline, twoside, openright, parskip,fleqn]{scrreprt}

\usepackage{soulutf8}
\usepackage{anyfontsize}
\usepackage{titlesec}
\usepackage{fontspec}
\usepackage{unicode-math}
\usepackage[numbers]{natbib}

\setul{0.3pt}{1.5pt}

\makeatletter
\newcommand{\ulex}[1]{%
  \let\@mkboth\@gobbletwo % neutralize \@mkboth in unnumbered chapters
  \expanded{\noexpand\ul{#1}}}
\makeatother

\titleformat{name=\chapter}
  {\normalfont\fontsize{14}{16.8}\bfseries}
  {\ul{\thechapter\enspace}}
  {0em}
  {\ul}
\titleformat{name=\chapter,numberless}
  {\normalfont\fontsize{14}{16.8}\bfseries}
  {}
  {0em}
  {\ulex}
\titleformat{\section}
  {\normalfont\fontsize{12}{15}\bfseries}
  {\ul{\thesection\enspace}}
  {0em}
  {\ul}
\titleformat{\subsection}
  {\normalfont\fontsize{12}{15}\bfseries}
  {\ul{\thesubsection\enspace}}
  {0em}
  {\ul}
\titleformat{\subsubsection}
  {\normalfont\fontsize{12}{15}\bfseries}
  {\ul{\thesubsubsection\enspace}}
  {0em}
  {\ul}

\setmainfont{Arial}
\setsansfont{Arial}

\setmathfont{GFSNeohellenicMath.otf}%equation font in arial font
\setmathfont[range=up]{Arial}
\setmathfont[range=it]{Arial Italic}
\setmathfont[range=bfup]{Arial Bold}
\setmathfont[range=bfit]{Arial Bold Italic}

\bibliographystyle{IEEEtranN} 
\bibpunct{/}{/}{;}{n}{}{,}
\renewcommand{\bibnumfmt}[1]{/#1/}

\begin{document}

\tableofcontents

\chapter{Thermodynamics}
\section{Heat}
Propagation of turbulent flames \cite{two} in spark-ignition engines is considered from the viewpoint of the different possible regimes \cite{three} of premixed turbulent combustion.\cite{four} Nondimensional parameters defining known combustion regimes are reviewed, 

    \medskip
    
    \bibliography{xyz}

\end{document}