Tsuki - Possession doesn't work properly

No matter what I try to do, whenever I start Tsuki, all that happens is that my screen resolution goes down and I get a black screen. I can’t even close the game, I have to restart my PC. I tried the recommended solutions, specifically this and this, but nothing works. Anyone knows what can be done, or should I open a ticket?

Usually this type of issue is related to needing DX9c installed.

I just download and installed Tsuki onto my Windows 11 machine and tested it. It started up fine, but I already had DX9c installed.

Well, I’m dumbfounded. I did download and install dx9c quite a while ago, since this isn’t the only 20+ years old game I’m playing. For some reason, Tsuki didn’t work before, but it ran just fine when I reinstalled dx9 now. Weird.
I had another problem with the game that I managed to resolve myself. Namely, the BGM sound setting doesn’t work, which made the music blast out at full volume, drowning out every other sound in the game. The problem is the MIDI format of the soundtrack, which lacks native support on Win10. So I had to download VirtualMIDISynth, boot a soundfont and manually lower the volume of the MIDI sound output. To achieve roughly the same volume between voices(set to max) and music, I had to lower the soundfont’s volume to 5% and in the mixer lower all the sound channels to minimum and the master volume to 40%. 
I hope this will be useful to anyone else having problems with old games with MIDI soundtracks.