or
Anonymous
In this MWE

tex
\documentclass{scrartcl}

\usepackage[spanish, activeacute]{babel}
\usepackage{unicode-math}
\usepackage{hyperref}

\begin{document}
\section{A section}
$$j =\sqrt{-1}$$
\end{document}


there is a fatal error. However, if I comment activeacute or \usepackage{hyperref} or \usepackage{unicode-math] or I change the kind of document to article, for example, the error disappears. Any help? Thanks 
Skillmon
I did not test thoroughly whether all the features work, but it seems like loading babel after unicode-math fixes the issue at hand.

tex
\documentclass{scrartcl}

\usepackage{unicode-math}
\usepackage[spanish, activeacute]{babel}
\usepackage{hyperref}

\begin{document}
\section{A section}
'a
$$j =\sqrt{-1}$$
\end{document}

Lualatex and unicode-math
Anonymous
Thanks for your interest.
Skillmon
@Anonymous Just found that there already is a related/duplicate issue in the bugtracker of unicode-math, therefore I didn't open another one. See: https://github.com/wspr/unicode-math/issues/462
Anonymous
But I think is a good idea report this problem to the maintainers of the unicode-math and babel packages. Can you do it? Thanks in advance.
Anonymous
@Skillmon In fact, I think the problem was activeacute as  Ulrike Fisher told me. I'm going to eliminate in my work. Thanks for your input .
Skillmon
@Anonymous did you also report this to the maintainers of those packages? If you didn't and don't want to, I'd do so.
Skillmon
@Anonymous Moving babel below unicode-math does work.
samcarter
@Anonymous To better understand your question, could you please add a small example why you need the activeacute option? Maybe there are other ways to achieve your desired output without it.