A recently upgraded my computer and on my old PC, this code
\RequirePackage{fix-cm}
\documentclass{scrbook}
\usepackage[no-math]{fontspec}
\usepackage{xunicode,xltxtra}
\usepackage{mathspec}
% Mathfont
\setmathfont{STIXGeneral}
\usepackage[german]{babel} % Neue Rechtschreibung
\usepackage{hyperref}
\usepackage[all]{hypcap} % Link to start of figures, not captions
\newcommand{\interpolationOp}[1]{\ensuremath{L_{#1}}}
\begin{document}
\tableofcontents
\section{Der Interpolationsfehler
\texorpdfstring{\( \|f-\interpolationOp{\mathbf{M}}f\| \)}{||f- IMf||}
}
\end{document}
Would produce both a correct PDF-Meta-TOC and a correct TOC using XeLaTeX.
However, now the TOC looks like this

Deleting either mathspec or the babel packes removes the faulty TOC, but I need both packages. Further, the .aux file states
\@writefile{toc}{\contentsline {section}{\numberline {0.1}Der Interpolationsfehler \( \delimiter "026B30D f-\ensuremath {L_{\mathbf {M}}}f\delimiter "026B30D \)}{1}{section.0.1}}
Which looks like TeXis expanding (but somehow just since I changed my computer) the \|, though the hint from here, declaring a robust command would not work.
Any ideas, why this is introduced in my new setup (though i can't say what the difference is, the old PC is not available atm, maybe a difference between MacTex 2013 and 2012)?
ngermaninstead ofgermanfor the new German orthography. – bodo Sep 20 '13 at 10:45