I have this same bug, and am looking for a solution. Any help would be appreciated.
I found that i had too much loot and left the tower, via the secret passage on the entry floor, to sell and make more room before i fought the boss. I didn't bother to open the gateway on the third floor, and now seem to be stuck. Is there any way around this? Because, i don't have a reasonable save file to go back to. If this bug can't be addressed, then i can't continue the game.