[Tilecache] Error generating tile at highest resolutions

[DFE]

Hello All,
I'm getting the following error when I'm requesting tile only at
highest resolutions, instead it works correctly for the lowest zoom
levels.

An error occurred: Current x value 1493284.913512 is too far from tile
corner x 1493272.954920
  File "/var/www/tlc/TileCache/Service.py", line 256, in modPythonHandler
    host )
  File "/var/www/tlc/TileCache/Service.py", line 181, in dispatchRequest
    tile = WMS(self).parse(params, path_info, host)
  File "/var/www/tlc/TileCache/Services/WMS.py", line 19, in parse
    return self.getMap(param)
  File "/var/www/tlc/TileCache/Services/WMS.py", line 27, in getMap
    tile  = self.getLayer(name).getTile(bbox)
  File "/var/www/tlc/TileCache/Layer.py", line 312, in getTile
    coord = self.getCell(bbox)
  File "/var/www/tlc/TileCache/Layer.py", line 287, in getCell
    raise TileCacheException("Current x value %f is too far from tile
corner x %f" % (minx, tilex))



I'm using OpenLayers 2.11:

 map = new OpenLayers.Map('map', { maxExtent: new
OpenLayers.Bounds(-20037497.21, -20037497.21,20037497.21,
20037497.21),
		 projection: new OpenLayers.Projection("EPSG:3857"),
		resolutions:[156543.0339,78271.51695,39135.758475,4891.969809375,2445.9849046875,1222.99245234375,611.496226171875,305.7481130859375,152.87405654296876,76.43702827148438,38.21851413574219,19.109257067871095,9.554628533935547,4.777314266967774],
		units: "m"});
var tlc_land = new OpenLayers.Layer.WMS("TileCacheMerc land",
		 "http://"+window.location.host+"/tlc/tilecache.py",
		 {format: 'image/png', layers:'land_merc'}
		);
		map.addLayer(tlc_land);

and I'm usign TileCache 2.11 under FastCGI module of httpd:

[land_merc]
type=WMS
url=http://localhost/cgi-bin/mapserv?map=/var/www/maps/main.map
layers=land_merc
extent_type=loose
# spherical_mercator=true
resolution=156543.0339,78271.51695,39135.758475,4891.969809375,2445.9849046875,1222.99245234375,611.496226171875,305.7481130859375,152.87405654296876,76.43702827148438,38.21851413574219,19.109257067871095,9.554628533935547,4.777314266967774
bbox=-20037497.21, -20037497.21,20037497.21, 20037497.21
srs=EPSG:3857
tms_type=google

Of course the direct WMS calls to mapserver is working correctly.

Do you have any suggestion?
Thanks in advance.

domenico febbo.