Justification broken

Question

I recently had problems with the text justification. Most of these have been resolved by the simple adding of the microtype package. However, I still have one broken justification in my text:

I really don't understand how to solve this problem. Here is my preamble and the example:

\documentclass[a4paper,12pt]{report}

\usepackage[top=3cm, bottom=2cm, left=2.5cm, right=2.5cm]{geometry} 
\usepackage{gensymb} % degree symbol in text

\usepackage{microtype} % line break justification fix

% exponent citation
\usepackage[superscript,biblabel]{cite}

% font 'utopia'
\usepackage{fourier}

% greek letters in text mode
\usepackage[artemisia]{textgreek}

\usepackage{float}
\usepackage{graphicx}
\usepackage[justification=centering,font=small]{caption} % caption center and small font
\usepackage{titlesec} % title sizes

% title sizes
\titleformat{\section}
{\normalfont\large\bfseries}{\thesection}{8pt}{}
\titleformat{\subsection}
{\normalfont\normalsize\bfseries}{\thesubsection}{8pt}{}
\titleformat{\paragraph}
{\normalfont\normalsize}{\theparagraph}{8pt}{}
\titleformat{\subparagraph}[runin]
{\normalfont\normalsize\itshape}{\thesubparagraph}{8pt}{}


% coloring refs
\usepackage[colorlinks=true,linkcolor=black,citecolor=blue]{hyperref}
\newcommand{\refc}[1]{{\hypersetup{linkcolor=blue}\ref{#1}}}

\renewcommand{\baselinestretch}{1.1}

\begin{document}

\section*{Aziridines}

Aziridines compounds can be exploited for the direct copolymerization with 
CO\textsubscript{2} into polyurethanes. This route is not the most explored and 
remains not well understood in all its aspects. A mechanism has nonetheless 
been proposed to explain the polymerization process.

\end{document}

Thanks for your help