I tried to reinstall this game, but it didn't helped.

PS. Since GOG refuses .jpg file I'm gonna describe that problem.

Some kind of prolonged black triangles are appearing on the screen(in game) (I guess this is caused by rendering). It's covering 1/3 of my screen.

You should provide more information about your system (hardware, OS, etc.) in order to get help.

Supposing that you are using Windows 10 or 11, I suggest you to try installing dgVoodoo 2.
That said, you could take a look here for possible solutions.