So I want to cure Raubsticht from his curse,
but I can't speak to Ellisia a second time.
The healer inside the church pointed me towards Ellisia again but she refuses to answer my questions, she even doesn't want to trade with me. Maybe she is upset that I got the longvity potion for the alchemist and I don't find her book which she wanted me to fetch.
What am I doing wrong?

Yes, Ellisia is upset with you for taking the longevity potion from her and will not answer any questions or trade with you until you have fetched the book for her. However, as she can answer many questions needed to complete many quests in Act 1 including the necessary question needed to cure the curse, she can be put to a better disposition to answer questions again if you retrieve the book for her and complete that quest. The book can be found outside the Iron Mines.

In terms of gameplay there is nothing wrong being done as it does not lead to any deadends. In terms of the story, Ellisia's warning to you that the alchemist is not of good character when you took the potion from her to give to the alchemist will become apparent at the very end of the game.