Next guess is that the extra middle stem on the space bar is rubbing against the switch.
To try to eliminate the stabiliser inserts, can you try removing them, then installing the space bar on the keyboard. Press the space bar ONLY IN THE MIDDLE (so that you do not place sideways stress on the switch or the space bar stem), and see if it still sticks. If it does, then it could be the extra middle stem rubbing against the side of the switch.