This is actually an issue with Renpy and complex sandbox games such as this one. It may not be the best engine for such games.
When DC adds a new storyline, it needs new variables to be created/set. To avoid Renpy crashes when an unassigned variable is read, they are usually initialized at the beginning of the game. However when you load a save, any new variable is not set, and then if you try to use them (being in a scene that tests for the variable to check for new events or dialogues) the you have a crash. There are ways to avoid such conditions, that were implemented for example in Bright Past, however for some reason DC did not implement it in this game.
It is stated in the OP that the saves won't work, and that's how the game works. If you are unsatisfied with this, then I suggest that you pledge on DC's Patreon, and then maybe you can request that he implements compatibility features. However don't be surprised that releases take more time for no additional content.
