Centering problem with beamerposter [closed]

Question

I use beamerposter in order to do a poster and I have some trouble because the contents of the poster is not centered. I tried to do a minimal example. This is the tex file :

\documentclass[final]{beamer} 

\usepackage[T1]{fontenc}

\usetheme{miniposter}
\usepackage[orientation = portrait, size = a0, scale = 1.4]{beamerposter}                       % e.g. for DIN-A0 poster

\usepackage{lipsum}

\title{Title of the poster}
\author[Author]{Author 1, Author 2 \ldots}
\date{2-5 Juillet 2012}
\institute[INSTITUTE]{Institute of wonderthings}

\begin{document}

\begin{frame}{}

\begin{columns}

\begin{column}[t]{0.64\textwidth}

\begin{block}{first block}
    \lipsum[1]
\end{block}

\end{column}
\begin{column}[t]{0.32\textwidth}
\begin{block}{a second block}
    \lipsum[2]
\end{block}
\end{column}
\end{columns}

\end{frame}

\end{document}

For beamerposter you need to create a kind of beamer theme. Here also I tried to put the minimum things just to see something.

\ProvidesPackage{beamerthememiniposter} 

\mode<presentation>

\setbeamercolor{headline}{bg=blue, fg=white}
\setbeamercolor{footline}{bg=blue, fg=white}
\setbeamercolor{author in head/foot}{fg=white, bg=blue}
\setbeamercolor{title in head/foot}{fg=white, bg=blue}
\setbeamercolor{block title}{fg=white,bg=blue}
\setbeamertemplate{navigation symbols}{}  % no navigation on a poster

\setbeamertemplate{headline}{  
  \leavevmode
  \begin{beamercolorbox}[wd=\paperwidth]{headline}
        \Huge
        \inserttitle\par

        \insertauthor\par

        \insertinstitute\par
  \end{beamercolorbox}
}
\mode<all>

This is the output, look at the block at the middle of the page. They are not horizontally centered. They are a little bit on the left side.

I tried some \centering or center environment or \hspace* but I never succeed.

Thanks for your help.