I'm using MikTeX 2.9. When processing the following:
\documentclass{article}
\usepackage{hyperref}
\usepackage[hyperref]{ntheorem}
\usepackage{aliascnt}
\newtheorem{lemma}{Lemma}
\newaliascnt{definition}{lemma}
\newtheorem{definition}[definition]{Definition}
\aliascntresetthe{definition}
\begin{document}
\begin{definition}
\label{def:first}
This is my definition.
\end{definition}
\begin{definition}[Alternative to \autoref{def:first}]
This is my second definition.
\end{definition}
\end{document}
I get:
! TeX capacity exceeded, sorry [grouping levels=255].
\HyRef@autoref #1#2->\begingroup
\Hy@safe@activestrue \expandafter \HyRef@au...
l.31 ...ition}[Alternative to \autoref{def:first}]
If I remove the autoref, all is well. This doesn't happen to me with amsthm instead of ntheorem. Am I doing something wrong? Is it an ntheorem bug? an aliascnt bug?
hyperrefshould be loaded last (as you've done). – barbara beeton Jul 21 '11 at 15:40