Closed Loop Stepper Motor

Dear Members,

I have a self build CNC router. I upgraded my Y-axis with a closed loop stepper to upgrade my working speed. The problem now is that I didn't get a circle out of it. It is allways a oval. The X-axis is a stepper motor. I also exchanged this one for testing but still the same issue. When I'm lowering the speed the error is smaller. Normal parameter is 2000mm/min.

I hope you can help me! I wanted to upgrade with the closed loop but I'm not sure anymore if it was an upgrade.

Thank you