you say that the floor is 40m2, that's a big room, is it 4mx10m, 5mx8m?
7"x2"joists seem a bit light for that size of room unless there is support walls, in the correct places, the quality of the tile shouldn't come into it, or the use of chipboard, i take it the tiles aren't boast? in my experience the better quality the tile the more brittle the tile, the main problem is movement, the fact that it still cracked even with ditra is surprising, there must be too much movement,
fill a glass of water, set it on the floor where there is the most movement and bounce on the floor, if if there is any more than mild ripples the floor isn't supported enough