Sakai-kun found that one of the arms of the translation stage was not moving properly, the step motor was turning but the long black screw wasn't. I found that the grub screw (see the picture) was loose. I checked the other arms and found another loose screw. I guess the company that cleaned and greased it forgot to re-tighten those screws. I tightened them all. Then, by using the Zaber consol, I reset the lockstep of the couples to make them move together again.