Slides with columns export problem

["Stéphane Genaud" <genaud@unistra.fr>]

Hello,
i am using org-mode 8.2.10 to export to a beamer tex source. 
When running pdflatex my org-mode source file, and once i finally isolated 
the problematic bit in a single frame, pdflatex says: 

[1{/opt/local/var/db/texmf/fonts/map/pdftex/updmap/pdftex.map}])
Runaway argument?
\let \AtEndDocument \@firstofone \@enddocumenthook \@checkend {docume\ETC.
! File ended while scanning use of \beamer@collect@@body.
<inserted text> 
                \par 
<*> simple.tex


The problematic frame is:
-------------------------------------------------------------------

** Kernel Roles
*** Roles 						:B_column:
    :PROPERTIES:
    :BEAMER_env: block
    :BEAMER_col: 0.5
    :BEAMER_envargs: <2->
    :END:
    - Program Execution
    - File Maagement
    - I/O Management
    - Communication
    - Protection & Security
    - Accounting
    - Resource Allocation
*** Image: What does it do? 				:B_column:
    :PROPERTIES:
    :BEAMER_env: ignoreheading
    :BEAMER_col: 0.5
    :END:

#+LABEL:fig:kernel-roles
#+ATTR_LaTeX: width=0.8\linewidth
#+ATTR_HTML: width="80%"
[[file:fig/kernel-roles.png]]
—————————————————————————————————

for which the tex-beamer export produces:

-------------------------------------------------------------------
\begin{frame}[label=sec-2-6]{Kernel Roles}
\begin{columns}
\begin{column}{0.5\textwidth}
\begin{block}{Roles}
\begin{itemize}
\item Program Execution
\item File Management
\item I/O Management
\item Communication
\item Protection \& Security
\item Accounting
\item Resource Allocation
\end{itemize}
\end{block}
\end{column}


\begin{center}
\includegraphics[width=.9\linewidth]{fig/kernel-roles.png}
\end{center}
\end{frame}
—————————————————————————————————

Here, a second \begin{column}…\end{column} is missing.
I have successfully used this construct in other presentations.
 
I have tried plenty of variations but i do not really understand what 
will trigger/not trigger the creation of a second  \begin{column}…\end{column}.
The org-mode example related to this page setup in the documentation 
(http://orgmode.org/worg/exporters/beamer/tutorial.html#orgdaf54b2) does not
explain much apart from the specific example given.

Any help to understand how the exporter works would be appreciated.

Thanks.
 
	—Stéphane.