had the same thing in one of my games. Can't provide a debug though. Sorry. Behavior after trying some was like: The game saved and played alright for some time then a gamecrash on save (had autosave each turn) and even loading older saves up to a certain point also crashed on save from the crash on but when I loaded back long enough the issue went away and reappeared some hours later (have just had that in one game so it might be something AI-Thrad specific possibly?). Went Quest of Mastery with this game since I had no motivation to further experiment with this annoying bug but can very much confirm it.

I just saw the same thing on one of my games.  I was relying on auto-saves so there goes those 3 hours I put into it.

Had this error multiple times before as well - and have submitted a bug report on it.

I really hate that bug, because it essentially it makes the game unplayable for me. It is really frustrating not knowing when the game will suddenly refuse to let you progress. I think the only solution is to disable autosave and hope to complete the game really quickly after that (before hitting another bug). As that is not an acceptable solution to me, I have stopped playing altogether and hope it will be fixed in Beta 4.