Hi Tiles (what's in a name
)
I've pinpointed down the problem to the acceleration-cache. Reduce its setting from "Optimal" to "None" and the tiling disappears.
I'm not sure if this problem is acceleration-cache specific or is due to combining acceleration-cache settings with image projection.
However, in future testrenders I recommend using higher quality rendersettings for more accurate results.
I saw you're using a renderdetail of 0.3 and I can tell you when you render your final image at detail 0.7 virtually everything will look different from your testrenders.
Also, the quality setting of your cumulus is insane. I'd reduce that 5-fold or so. I saw when doing that the color of the clouds changes dramatically but you can easily fix that by adjusting the cloudcolor in the cloudnode. In final renders this would add serious amounts of rendertimes (5x times longer or so).
Just my extra 2 cents
Martin