[mapserver-users] Mapcache error: tile y=2 not in [0,2[

[mapserver]

I am building a public mapservice for the city of Harare, where the background 
map is served through tilecache (mapserver 6.4.1/tilecache 1.2.1).

http://197.211.215.14/water/weekly_supply

is almost functional (maybe you have to zoom in/out at first) What is bothering 
me are the two pink tiles in the upper border at 1:400000, which show the message

  tile y=2 not in [0,2[

in the apache error log. The tiles to the left and right of the pink ones are 
served o.k. (They are blank anyway)

Could somebody tell me what's going on there?

Thanks - Markus