I’ve found something,
if I make spine_05 larger (the shape / bone onto which the breasts are attached), in such way that both breasts are inside, it acts strange for a little second, and afterward it stabilizes.
Maybe I’m on a solution here if I render it with pre-frames.
Another thing that seemed to work is to right click spine_05 and “regenerate bodies”.
It makes the shape very little.
Eather way, it’s not colliding anymore with the breasts, and that way the chest is stablized,… I guess.
Now another thing that would be nice is to scale the shape around spine_05, without locked proportions… I don’t seem to find where to do it.