[OSM-newbies] Tirex/Mapnik does not deliver all tiles

[fatzopilot]

First of all, sorry for reposting, I tried via nabble first but is seems  
the mailing list is not accepting those nabble messages.

Working through the docs in the wiki, I finally got a mapnik/tirex stack  
(on-demand tile generation) up and running except some showstopper:
Some tiles are not delivered by mod_tile. E.g.  
http://tileserver.myosmserver.org/tiles/osm/1/1/1.png generates a  
meaningful tile (and the corresponding meta-tile is generated) but  
http://tileserver.myosmserver.org/tiles/osm/2/1/1.png results in a 404.
However, the rendering process is started in all cases but once it  
finishes, the 404 message is returned and no (meta-)tile is generated.
http://tileserver.myosmserver.org/tiles/osm/10/1/1.png works again but  
just shows an empty tile, i.e. ocean.
All tirex test run successfully and mapnik is able to generate a map of GB  
(generate_image.py). Generate_tiles.py also works flawlessly and  
meaningful tiles are generated, e.g.  
/var/lib/tirex/tiles/osm/15/16356/10991.png.
Trying to access it via  
http://tileserver.myosmserver.org/tiles/osm/15/16356/10991.png again  
results in a 404. Mapnik 0.7.1 and Tirex 0.5 (latest svn) are both  
installed from self built packages and run on Ubuntu 10.04 (64).
Any clue what could be the problem here? Hints welcome!

Thanks
fatzopilot