My suggestions for this fix:
Scrap the current version to its most stable one
Update the save system
Update the game from Unreal Engine 4 to 5, if not done already
Make sure to COMPILE your game, both before making updates, during the process of making them and after, because this is how MOST GAME-RELATED PROBLEMS are FIXED
Stick to a single engine, this is much easier to do with UE5 and is part of why I recommend it, as well as the fact that it has the blueprint system
Read sources that contain how to fix particular issues that pop up in logs
Note that this is coming from a gamedev learning about gamemaking from people who run their own gamedev studio and are funded by the government to teach people such as me, I'm also up for helping where I can to help speed up the processes of fixing bugs or helping with development