Das hier funktioniert bei mir (british -> english und classicthesis-preamble auskommentiert):
\documentclass[english,fontsize=11pt,paper=a4,twoside,openright,titlepage,numbers=noenddot,headinclude,BCOR=5mm,footinclude=true,cleardoublepage=empty]{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[utf8x]{inputenc}
\setcounter{secnumdepth}{2}
\usepackage{varioref}
\usepackage{prettyref}
\usepackage{tipa}
\usepackage{amstext}
\usepackage{splitidx}
\makeindex
\usepackage{graphicx}
\usepackage[authoryear]{natbib}
\newindex[Index]{idx}
\newindex[Author Index]{aut}
\newindex[Language Index]{lan}
\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
% Classic Thesis Style loader
%\usepackage{classicthesis-preamble}
% use Latin Modern instead of Computer Modern sans serif
\renewcommand{\sfdefault}{lmss}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
\usepackage{arabtex}
\usepackage{blindtext}
\makeatother
\usepackage{babel}
\begin{document}
\chapter{General characteristics of the lateral approximant}
\section{Articulation of the lateral}
\subsection{Tongue tip and blade\label{sub:Tongue-tip-and}}
\blindtext
\bgroup
\addtocounter{subsection}{-1}
\let\thetmpsub\thesubsection
\renewcommand{\thesubsection}{\thetmpsub~1/2}
\subsection{Tongue tip - tongue body coordination}
\egroup
\subsection{Tongue body}
\end{document}