Game updates often change the internal structure of the game by adding new content or altering existing mechanics. Saved files store information about the current game state but are based on old data. If the game tries to use an old save after an update, data mismatches can lead to crashes, freezes, or incorrect behavior, as the new version expects a different data structure. This makes old saves incompatible.
Mate... start listening to your playerbase. Noone wants to replay the game over and over again. It just feels like a punishment for playing the game before its finished - not a good way to grow yourself a community / playerbase.
This game is very straight forward and has no real differing choices. You just complete quest after quest. Meaning, you could just add the latest gamesave of the current version of the game. Or change your code to solve this reoccuring problem.
Don't awnser with excuses when everyone knows(!) that there are ways to manage this. Other games do it too. With the same engine.
There are multiple ways to solve this problem, but until now you have decided against that. Thats a real shame. Your game has a solid idea and foundation, but it's developer seems to lack the respect for the players. I would like to be proven wrong, but so far this is how this situation looks to me. It's not always easy, but keeping a solid playerbase (even if its small) is the key to growing bigger. Especially in this specific genre. And therefore punishing the players (no matter if intended or not) does not help.