I tried again with a new WAV file generated with CDex. The results are "better" in a way. Now the picture of the second load screen looks much cleaner (some corruption on the lower edge) but the music is completely garbled.
At least the game seems to start but I'm still not entirely happy. I didn't expect this to be such a delicate process.
I'm always happy about any pointers as to what I could try. Maybe it will even help other people. Thank you.