Circles not round (no replies)

I have a coreXY printer (Hypercube Evolution)

Today I found out, that shapes prints are distorted:
A circle is not a circle, but wider along the Y-axis than along the X-axis, by half a millimetre.
Where could this problem come from?
The belts are tensioned. The angles are 90deg. Both motors for XY movement are identical and have the same steps/mm set in config.

I am running out of ideas.