On investingating further, it looks like you chose to fight on past the required enemies then quit while still in the dungeon. There was a load order issue in V45 (and possibly also a few versions before that) in that specific situation.
The good news is that crash was fixed along with a big combat load order rework in V46 some months back. The bad news is to save that character before V46 you'll need to do some light save editing, which I can guide you through. Mostly just deleting the last few kill stats so it rolls you back to the chest.
Edit: Or you can email me the save or drop it in the discord and I'll fix it.