it might be because of the item, i can't be sure without knowing the exact error that appeared, because switching sides is the mechanic to losing a girl in that fight.
the notes are right in front of the middle of the office table
View attachment 3935246
so far there's only the bad end for having all maidens fully corrupted if you lose the final fight, eventually you simply won't be able to flirt with them anymore and they may try to corrupt other girls randomly (not yet implemented)
turns out I forgot to add the scene for going to the rescue after unlocking the succubus fight. I'll have it fixed for next update