Hi there, Did you tighten the Ball Screw Coupling Set Screws? There are 2 on each coupling, one for the motor and the shaft. You also need to make sure the screws are on the flat part of the shaft. (one for each axis)
Wow guys, that was a quick response! I just solved it thanks to all three of you.
Powered up, push and pull: there was still some movement of the gantry on left Y-axis. I tightened the screws on the Y-motor (see picture) onto the ball screw and that solved the problem. Thank you so much, what a great community.