Son of Durin (and other folks),
I'll try and make a clear post on how to do this save fixing approach I've been using. It'll be a wall of text to explain what to do and why to do it but it might help folks that are determined to do so.
I'll give a massive hint though... this particular error seems to result in either the VN's stat bar or the entire in-game UI.
One will notice that you can almost tell immediately if a save file is bugged as upon load the VN's stat bar at the top of the screen will show then hide immediately... that was my original first clue when the VN updated and this particular save file issue was being reported with folks modded or unmodded.
One of the steps in the middle of my overall repair process is to open the MiniMenu and toggle "SanchoFont" Off/On (or On/Off depending on it's current state) which is just a quick and easy way to force RenPy to refresh the entire UI. Originally I had some code in that new "MainMenu" QM button to do a UI refresh but the actual time it took for the refresh to complete was abnormally long compared to using an embedded style registry toggle such as is contained in font changing, etc.
This post is already becoming a wall of text and that particular step I just wrote is not the first step nor the last so I'll stop there but thought I'd share a bit of insight. Again, I'll see what I can do to write up a clear step by step process that will help resolve this particular error (it will do nothing for rollback errors, that's a completely different issue).