[MapProxy] mapnik source config for OSM

[lionel b]

Hi,

I try to configure a mapnik source.

- I installed mapnik on a debian squeeze and I can play 
generate_tiles.py succesfully with an OSM postgis db.
Mapnik version :
dpkg -l | grep mapnik
libmapnik0.7                    0.7.1-3
python-mapnik                   0.7.1-3

- I installed mapproxy (1.2.1) and I configured my source like this :

layers:
   - name: osm_mapnik
     title: OSM mapnik
     sources: [my_osm_mapnik]
.........

sources:
   my_osm_mapnik:
     type: mapnik
     mapfile: /my/path/to/osm.xml

if i try to display 
http://localhost/cache/demo/?wms_layer=osm_mapnik&format=image%2Fpng, 
nothing appends (request aborted after a very long time wait).
I can display other layers (wms sources) without any problem so I think 
my mapproxy install is ok.
I can't see any error neither in apache logs nor in mapproxy.log
Did I miss something important ?
Regards,

Lionel