https://www.nexusmods.com/pillarsofeternity2/mods/2 (does not disable achievements)
Download it, enter conversation with the NPC that's giving you problems, and on the right side you will see an option to ''Debug Dialogue''. Try and find the correct dialogue choice so you can progress your quest. If that doesn't work, then I'm afraid you're gonna have to wait until an official fix comes out.