Thanks again Julius,
I already had that connection there before I made the group on that problem joint, but it looks like groping cuts the link (even if it is still visible when you select the joint).
It works perfectly when I start by making all groups, and then make all connecting of joints after that. (Doing it the other way, connecting joints and then grouping, doesn’t work).
I also realized that if you want to connect multiple objects, so that they move together, the logic to connecting two boxes is to group the joint to one and to connect it to the other.
To make the UI on this connecting more intuitive I suggest making it possible to connect and group joints in a random order. Also making the joint “un-static” should make it move together with the objects it is connected to, without the need for groping the joint to the other object.
Sheik