What I don't know if it is intended or if it's a glitch with the dialogue options as they are stuck in an endless loop (with the not nice sounding answer making her angry and asking you to not talk about it again).
I can understand her not wanting to turn if you were a [censored] to her during the Dawnguard questline but if you were nice to her for the duration it seems weird for her to blow you off like she does.
So is it a glitch? Or will I have to svck it up, load and old save from 7 or 8 hours ago and do the Soul Cairn, Vale and the final quest again to see if it works?