Ah, okay.
Is the center of the "havok radius" at 0,0,0 coordinates in the cell? Because all my objects are placed on the negative side of one axis (the closest to 0 being -250), and expanding further in that direction causes the error to appear. So does this mean that I can expand the cell an equally long distance in the opposite direction, without triggering the message again? (my errors start appearing somewhere beyond -15000, so placing things at 15.000, would that work?)
Yeah, try to start things at 0,0 so you have the most available room. However I'd agree with the others, either split it into two cells, roombound it well (if it's within the havok bounds at 0,0) or convert it into a worldspace.