(Incomplete \iffalse) with enumerate and Hebrew [closed]

I get the same error as reported here. An update of miktex did not help so I am at a loss as to how to proceed. The example works just fine on a different machine.

\documentclass{article}

\usepackage{multirow}
\usepackage{ntheorem}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{cancel}
\usepackage{tikz}
\usepackage{fontspec}
\usepackage{polyglossia}
\usepackage[shortlabels]{enumitem}

\renewcommand{\theenumi}{\hebrewnumeral{\value{enumi}}}
\renewcommand{\labelenumi}{\theenumi}

\theoremstyle{nonumberplain}
\setmainlanguage{hebrew}
\newfontfamily{\hebrewfont}{Arial}

\newtheorem{thm}{משפט}
\newtheorem{prop}{טענה}
\newtheorem{lem}{למה}
\newtheorem{defin}{הגדרה}
\newtheorem{rmrk}{הערה}
\newtheorem{cor}{מסקנה}
\newtheorem{proof}{הוכחה}
\newtheorem{ex}{תרגיל}
\newtheorem{expl}{דוגמא}

\begin{document}
\title{שלום עולם}
\maketitle
\section{ניסיון}

%%\begin{thm}
%%גרף מישורי מקיים
%%$$%%m \leq 3n-6 %%$$
%%\end{thm}

%\begin{enumerate}
%\item
$$a_{i}^{'}=\begin{cases} a_{i}& i \neq t\\ 2t=b_{t}& i=t \end{cases}$$
ו-
$$b_{i}^{'}=\begin{cases} b_{i}& i \neq t\\ 2t-1=a_{t}& i=t \end{cases}.$$
%\item
$$a_{i}^{''}=\begin{cases} a_{i}& i \neq t+1\\ 2t=b_{t}& i=t+1 \end{cases}$$
ו-
$$b_{i}^{''}=\begin{cases} b_{i}& i \neq t\\ 2t+1=a_{t+1}& i=t \end{cases}.$$
%\end{enumerate}

\end{document}

• You shouldn't be using $$...$$, but $...$. I get no errors after removing the % characters; more information about the system where XeLaTeX fails is necessary. What version of MiKTeX is running on it? – egreg Mar 24 '14 at 13:11
• @egreg I tried [ but get the same error. ($$..$$ worked just fine on the other machine). Miktex is 2.9. What could be going wrong here? – Felix Goldberg Mar 25 '14 at 14:57
• Add \listfiles and compile on both machines, then compare the lists to see where's the mismatch. – egreg Mar 25 '14 at 15:08
• I didn't get any error with your example. Using MikTeX 2.9 (but not recently updated). Is it possible that some recent behavior change caused this? What other OS+TeX distro combinations have you tried? ... Also, please try to further reduce your example. – einpoklum Mar 25 '14 at 17:32