There is a flexible, transparent plastic plate in the tube, at the level of the hinges, which is supposed to take the pressure when the hinge is closed. This plate can come loose and slide into the tube, which hinders or prevents the movement of the support bar in the tube. I was only able to solve the problem by using brute force, which meant moving the plastic plate to a place where it was less in the way, but it's not a good solution: the hinge doesn't close any more and the bar is still difficult to move. You'd have to be able to dismantle the support bar completely, but I haven't found out how to do that.

Make sure that the handlebar sides are parallel, then try gently tap on the end of it it might help to unlock it.