As the error message states tabularx is not defined, so you need to load tabularx. Also there were some accented A in your page title file, look like a mis-encoding of utf8 no break space. I removed those and also added \noindent before the tabularx so that you can get a full width table (otherwise the line is over-full by 15pt = the paragraph indentation). Also use \input not \include.
\documentclass{report}
\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[francais]{babel}
\usepackage{tabularx}
\input{pagetitre}
\title{Text}
\author{Text}
\sujet{Text}
\motsclefs{Text}
\datedebut{September 26th}
\datefin{December 3, 2005}
\superviseurs{Name1 \textsc{Blabla} (ACRONYM)}
\proverbe{A leader is someone who steps back from the entire system and tries to build a more collaborative, more innovative system that will work over the long term.}
\auteurproverbe{Robert Reich, United States Secretary of Labor (1993--1997)}
\begin{document}
\maketitle
\paragraph{Lorem ipsum}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Curabitur vestibulum risus nec dapibus aliquam. Phasellus ultricies egestas erat sed mollis. Etiam euismod tortor ut ante rutrum consequat. Nunc id sem convallis, iaculis diam quis, facilisis nulla. Aliquam leo lorem, ultricies.
\end{document}
While I still can't pinpoint the source of the problem, removing the titling package resolves the issue. It appears to be a package clash, but with what—I do not know yet.
Best Answer
In the standard classes: no. In the
KOMA-Scriptclasses: yes.