I managed it :-) My mistake was that I didn't format DATA, now 9.8.15 runs fine.
In case others have similar problems read:
https://www.android-hilfe.de/forum/root-custom-roms-modding-fuer-xiaomi-mi9-se.3550/root-anleitung-bl-unlock-und-flash-der-xiaomi-eu.910604.html (German language).