Ok, sono riuscito a capire che il problema insorge a causa di questo script per otenere la pagina di titolo (inserita poco dopo il \begin):
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\documentclass[a4paper,12pt]{article}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english]{babel}
\usepackage{amsmath,amssymb}
\usepackage{graphicx}
\usepackage[font=small,format=plain]{caption}
\usepackage{subfig}
\usepackage{wrapfig}
\usepackage{epstopdf}
\usepackage{booktabs}
\usepackage[verbose,a4paper,tmargin=20mm,bmargin=25mm,lmargin=25mm,rmargin=20mm]{geometry}
\usepackage[output-decimal-marker={,}]{siunitx}
% This is all for formatting and making the Table of Contents according to
% spec (DOTS). Don't play with it.
\makeatletter
\renewcommand\l@section[2]{%
\ifnum \c@tocdepth >\z@
\addpenalty\@secpenalty
\addvspace{1.0em \@plus\p@}%
\setlength\@tempdima{1.5em}%
\begingroup
\parindent \z@ \rightskip \@pnumwidth
\parfillskip -\@pnumwidth
\leavevmode \bfseries
\advance\leftskip\@tempdima
\hskip -\leftskip
#1\nobreak\
\leaders\hbox{$\m@th\mkern \@dotsep mu\hbox{.}\mkern \@dotsep mu$}
\hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
\endgroup
\fi}
\makeatother
\setlength{\parindent}{0pt}
%————————————————–INIZIO DOC
\begin{document}
\renewcommand{\rmdefault}{ppl}
%———————————————–TITOLO
\begin{titlepage}
\begin{center}
\textbf{\Large{} Politecnico di BlaBla }\\
\textbf{\Large{} Dipartimento di Ingegneria }{\Large }\\
{\Large{} Corso di Impianti }\\
{\Large{} }Prof. Tizio Caio \vspace*{10\baselineskip}
\par
\end{center}
\begin{center}
\textbf{{\Huge Pressurization System Analysis }}\\
{\Huge{} \vspace*{0.2\baselineskip} }{\large Mario Rossi }\\
{\large{} \vspace*{10\baselineskip} \today}
\par\end{center}{\large \par}
\end{titlepage}
%———————————————————-FINE TITOLO
L'Adige è lungo \SI{410}{\kilo\metre}, il Tevere \SI{405}{km}.
\end{document}%—————-FINE DOC
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