I also put this question in the Scipy discussion list,
and here the following answer finaly solved the problem:
Matthieu Brucher wrote:
Are you using MAtplotlib with the wxAgg backend ? If it is, you should use the wx one,
*wxAgg is no longer supported with wxPython 2.8*
So after changing MatPlot backend from wxAgg to wx ,
(which is unfortunately much slower and more uglier)
the error still appeared.
After removing all files _wxAgg.*,
the problem was solved.
cheers,
Stef