I'm not certain there is a "fix" outside of going back to an actual manual save before starting the quest. There is a thread on reddit claiming to have a fix. It relies on the [not fully functional] save game editor. Unless you are only changing specific aspects of the save, it's possible this method will overwrite information you don't want to lose.
I'm going to link it in the hopes its info will be enlightening, but first I want to put a big ass disclaimer...
USE THIS METHOD AT YOUR OWN RISK... The results may end up worse than the start. And for the love of Captain Picard, backup your installation and Fallout 4 save/config folders before you EVER modify, move, or even look cheekily at them.
https://www.reddit.com/r/Fallout/comments/3tgufy/confidence_man_immortal_bug_fix_100_fix_for_pc/