I was wondering if anyone could have a clue on what is happening with this particular character. For some reason I get constraints jumping out of place at simulation time and this happens instantaneously without any collision. This also happens both in the physics asset editor and in the scene. It also seems to happen only on the right half of the character. A more extensive description with animated GIFs is available here: Constraint jumping on simulate - UE4 AnswerHub
Usually happens if there’s a collision overlap between joints. You could try toggling on *ignore parent. *That’ll only affect the relationship between the two objects on the same constraint and won’t apply further down the chain. But it makes elbow joints behave a lot better.