Resolution forced? If yes, try disabling DirectXExt\RTTexturesForceScaleAndMSAA.

I turned this off and ... the screen stopped showing (although I can hear the sound) xD

It's a dangerous option... 😀 But, is the problem present with unforced resolution too?

The problem with shadows is also when the resolution is set to "unforced"

Ok, then I'll have a look at it later.

I've just check out this game, but I cannot reproduce this issue. No matter if resolution is forced or not (I tried 2x upscale from 1280x720 to 2560x1440).
Does it happen at a certain area in the game? I only played the game to the quiddich part.

Anyway, I found an other problem though. When resolution is changed in the game menu, only a black screen what is got. I fixed this at least.

Er, there are no Quidditch games in this game, nor even the stadium Harry usually plays Quidditch at. Which game did you test? I tested this game - https://archive.org/details/hpootp_202110
As for the problem, it is in the game all the time, and for the first time in this game you can see it learning spells, specifically learning the Reparo spell.

Yes, it's the same game. One of the characters talked about quiddich and I played the game till that point, but anyway, I'm at the school yard (or sg like that):

This is upscaled from 1280x720 to 2560x1440. I had to run the game with GF 9800 card because the default supports paletted textures and the game has problems with them but anyway I cannot see anything wrong with the shadows.
Maybe I have a different version (patched) of the game? Or you set sg in the dgV config causing this?

This is the configuration I use for this game. I have dgVoodoo2 in the latest version, and the drivers for my graphics card are also the newest

The forced 8x msaa is the culprit.