I tested this earlier tonight and it was working perfectly fine. I'll need a save with the issue happening to look into it any further.
Alright, here. I just ran into the same thing.
Specifically, when you feed her the Incubus Draught, the game goes to a blank screen and there are no buttons down the bottom. Escaping back to the main screen via Stats, I found that Tifa was still under Slaves, but clicking on her button merely made the text in the window disappear - you stay on the Slaves menu.
Save attached. I'm using Firefox, if it helps.
View attachment CoC_2.sol