This is an accuracy problem of the display only (arcs and ellipses are interpolated with small line segments to be displayed for performance reasons). The constructed ellipses are precise (see property editor). Calculations with ellipses are precise as well.

Ah, I can see it from circumference/length, but it gives really unpleasant result. For example, red ellipse crosses axes in
[-100,0], [0,99.97], [99.98, 0], [0, -99.96].

Is there any way to increase number of line segments displayed?

Regardless precision, I am not happy about construction. EP hor ver produces different appearance from EP ver hor.

Thanks!

Is there any way to increase number of line segments displayed?

No.

EP hor ver produces different appearance from EP ver hor.

This is because the interpolation starts at different angles. The situation can be improved for ellipses with the same start angle and full ellipses by applying this minor patch:
https://github.com/qcad/qcad/commit/f2643aff45c018a952efbd4948e911a97145e077

This is answer I am looking for, as it will improve ellipses visual appearance.
/**
* Exports an ellipse with the current attributes.
* \todo switch from line based interpolation to arcs.
*/

Regards!