Hmm, well i was a little imprecise there, by extent i of course "meant" the cell-count, i think something around 200.000 pixels may be the threshold. You even see it if the tool starts, when it works, you can see the numbers "counting" in the processing-window. I assume you used elevation-models bigger than that?!
i also tested with a esri-grid 8bit signed int file, no differences and i resampled the raster to a lower resolution with the solution that the working extent grows.. 🙂
the only idea i have right now is to test this on another windows, maybe a non-x64 one.. but i think i should continue this in the other thread, i hope this will get a bit more attention. 🙂