does anybody know, what should I do to make the shadow smoother?
Use Eevee Legacy. Sun shadows are confirmed broken in Next, have been for years
If you are fine with the shadow being a bit blurry, you can go in the sun’s light settings and increase its “angle” setting to create soft shadows.
I made this in blender 5.2 and I couldn’t open it in 4.1
do you happen to know if we can use eevee legacy for 5.2?
No, this is not possible. Eevee is deeply integrated into Blender itself, so you can’t simply port the old Eevee into current Blender. If you wanted to bring this in 4.1, you would have to instead export the models and reimport them in 4.1.
But maybe there could be an other way. Can you describe what you are trying to do? Is this meant to be a realistic or stylized render? Do you need to do this in Eevee?
Try this