Hihi, doll in the chat here. Minorly spoilerish(?) for anyone reading who hasn't beaten Lethe's boss yet,
Are you stuck at, like, right after where the labyrinth gates open up? And the black rabbit says something along the lines of 'rule breakers like you aren't allowed past these gates'? If so, (and you got Mysterian as your class) you unfortunately cannot use that avenue to progress, you must instead backtrack to BF5 and march into the central courtyard (Ferdinand) where you'll meet Lilim, who will give you the key to progress. Apologies if this message finds you too late and you already figured it out lol