Too many math alphabets used in version normal error when using chemmacros package [closed]

Question

When I load package chemmacros, suddenly the following error occurs:

Too many math alphabets used in version normal.

I've already made sure that I've updated my MikTeX distribution.

Edit: my code is

\documentclass[english,fontsize=11pt,paper=a4,twoside,openright,titlepage,numbers=noenddot,headinclude,BCOR=5mm,footinclude=true,cleardoublepage=empty]{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\setcounter{tocdepth}{3}
\usepackage{amsmath}
\usepackage{amssymb}

\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
% Classic Thesis Style loader
\makeatother
\input{classicthesis-config.tex}
\makeatletter
% use Latin Modern instead of Computer Modern sans serif
\renewcommand{\sfdefault}{lmss}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
\usepackage[minionint]{MinionPro}
\usepackage{chemmacros}

\makeatother

\usepackage{babel}
\begin{document}
\[
-\frac{\hbar^{2}}{2m_{e}}\underset{i}{\sum}\nabla_{i}^{2}
\]

\end{document}

, whereby the "classicthesis-config.tex" is the same one as the one in https://code.google.com/p/classicthesis (LyX version).

What should I do to repair this problem?

By the way, if I replace the \hbar with \alpha, the problem disappears. The problem still remains if I replace \hbar with \hslash.