Are you playing with mods like urm? Those will corrupt the game state, yielding weird quest lockups at times.
Maybe you forgot to report back to the mirror? That might also be a reason the quest does not switch?
There are three floors and the third one is required to progress in the game. (the level with the spectral slime)
If you can send me a save game I'll analyze it.
If you are a patron you can post it on the discord bug channel.
If not, just put it up on some cloud drive (mega etc.) and send me the link here and I'll have a look