I have to say there is something tricky here:
You are supposed to go inside Mardaneus house from a cave in the begining of game to help him, I tried this again but strangely I can't now.
I readed the solution here, it worked (thanks!) but I tried to go into the cave then to check. You can go into the cave but the stairs were removed.
Come on Larian
Yeah I like to dig up OLD topic.