[MapProxy] blank tiles at some levels

[lionel b]

Hi,

I seeded a cache at different levels. If I use the demo page, I can see 
the result. At some levels, mapproxy send blank tiles.
level 0 : ok, I can see images in the demo page or with a WMS client
level 1 : ok
level 2 : ok
levels 3,4,5 : mapproxy send blank tiles
level 6 : ok
level 7 : ok

If I look directely in mapproxy cache in /var/cache_data/, I can see 
images well generated at levels 3,4,5

I can see this error message  in apache error.log but I don't know If 
it's related (comming after blank tiles) :

[Mon Mar 07 11:20:36 2011] [error] Error in atexit._run_exitfuncs:
[Mon Mar 07 11:20:36 2011] [error] Traceback (most recent call last):
[Mon Mar 07 11:20:36 2011] [error]   File 
"/usr/lib/python2.5/atexit.py", line 24, in _run_exitfuncs
[Mon Mar 07 11:20:36 2011] [error]     func(*targs, **kargs)
[Mon Mar 07 11:20:36 2011] [error]   File 
"/usr/lib/python2.5/logging/__init__.py", line 1354, in shutdown
[Mon Mar 07 11:20:36 2011] [error]     h.flush()
[Mon Mar 07 11:20:36 2011] [error]   File 
"/usr/lib/python2.5/logging/__init__.py", line 731, in flush
[Mon Mar 07 11:20:36 2011] [error]     self.stream.flush()
[Mon Mar 07 11:20:36 2011] [error] IOError: sys.stdout access restricted 
by mod_wsgi
[Mon Mar 07 11:20:36 2011] [error] mod_wsgi (pid=23458): Exception 
occurred within sys.exitfunc().
[Mon Mar 07 11:20:36 2011] [error] Traceback (most recent call last):
[Mon Mar 07 11:20:36 2011] [error]   File 
"/usr/lib/python2.5/atexit.py", line 24, in _run_exitfuncs
[Mon Mar 07 11:20:36 2011] [error]     func(*targs, **kargs)
[Mon Mar 07 11:20:36 2011] [error]   File 
"/usr/lib/python2.5/logging/__init__.py", line 1354, in shutdown
[Mon Mar 07 11:20:36 2011] [error]     h.flush()
[Mon Mar 07 11:20:36 2011] [error]   File 
"/usr/lib/python2.5/logging/__init__.py", line 731, in flush
[Mon Mar 07 11:20:36 2011] [error]     self.stream.flush()
[Mon Mar 07 11:20:36 2011] [error] IOError: sys.stdout access restricted 
by mod_wsgi

my demo url : 
http://194.167.15.228/mapproxy/demo/?tms_layer=orthophotos&format=jpeg&srs=EPSG%3A2154

Thanks,
Regards,
Lionel