While I was replaying the complete game once again, I got it too, but as mentionned by mindricofox: The problem was solved by disabling more_travelers.esm, it seems we may enable it after the dialogue again without side effect on the rest of the quest. From the above exchange, it seems that the issue comes from mods that are modifying dialogs, so if more_travelers.esm if not doing the trick for you, you may try to disable all your mods to see it comes from a conflict (and enable them again after passing the dialogs with daedric prince).