I encountered this problem too.
I tried this act a couple of times. I did finish it today, doing the same things I did before, just skipping dialogues and in order.
I think the problem was with Frippone - I'm not entirely sure, but I think I didn't get the last dialogue, about sitting on the bench.
I think previously I did not do the things related directly to Morglot's problems in order. I can try to reproduce it, although I don't know if I can retrace my steps.