I've checked my older saves, and in my case that particular sidequest could be completed after the Ardan Succession crisis, in the second "open world" section of chapter 4.
Obviously you need to have spoken to Bertricia before, learning that she wishes to know more about Ivalan succubi. That conversation can be held right away in early chapter 4 (the initial "Gather the Party" section). At some point after that an Ivalan succubus nun will show up outside Helvanna's cathedral: https://a.pomf.cat/fzasho.png
The exact prerequisites aren't clear to me - it might have to do something with social score. At that point in time, I had Social = 40 (which is also a threshold for various other things) and Religion = 24.