I finally found how to fix the Gantt chart issue completely. It IS a bug of version 2.0.1 The solution comes from here and is the following (original text preserved):
open
modules/tasks/gantt.php
an modify line 137 like down below for instance until I have sorted out with the concnering developer what to do:// This configuration variable is obsolete = line 137 $jpLocale = dPgetConfig( 'jpLocale' ); if ($jpLocale) { $graph->scale->SetDateLocale( $jpLocale ); } //$graph->scale->SetDateLocale( $AppUI->user_locale );