Section is redefined with a classical:

\renewcommand\section{\@startsection{section}{1}{\z@}
{-3.5ex \@plus -1ex}
{ 2.3ex \@plus .2ex}
{\bfseries}
}

Suppose you’re using an unnumbered section \section*. Everything is working fine, until you need to use mathtools for displaying an equation. And now, your unnumbered section is numbered, and its title is *...

The problem is just, you just put a new line between the }} where it should not have been. Just rewrite your definition, and it’ll be working again:

\renewcommand\section{\@startsection{section}{1}{\z@}
{-3.5ex \@plus -1ex}
{ 2.3ex \@plus .2ex}
{\bfseries}}