Hi! I've joined specifically to confirm the case with mods because I also had to google this. In my case, I'm using a few mods that modify the dialogue for Gus, Wizard, Willy and make them romanceable characters. One of the mods, "[CP] looking for love" also modifies Sandy.
So basically, what needs to be done is to delete any mod that changes the dialogues of vanilla characters involved in the quest. Any CP mod can be just temporarily moved, but for xnb mods that directly replace original game files, really to just delete them and verify the integrity of the files via steam - all vanilla files replaced with any xnb will be back. In my case, there were 14 that had to be reacquired. After those characters are restored to vanilla lines, the quest completes without a single error.
PS I'm posting here because this is the thread that pops up when I google the issue. I figure adding more info for other players who may be having these issues won't do any harm :) Good luck!
PS 2 I guess this means completing this quest might be impossible if married to the wizard on a modded game