Answer:
Instability is caused by the fact that all members with a hinge at the member end converge in one node. This means that the node can rotate freely. It is necessary to connect at least one member to a node without a hinge.
You could also solve this problem by setting a certain small stiffness at the hinge, so that it is not freely rotatable. In that case, there can be hinges on all members at the node.
