i started to enable mods and replicate the issue:
it seems as though the improved build menu mod was updated, and added a new dependency of 'Item Tweaker API'
i loaded the dependency and activated all mods. I played a character and could not replicate it
So it seems that:
1. this is mod related.
2. Most likely an updated to "Improved build menu" mod broke the game, as it added a new dependency that was not being detected at game start?
3. The game failed to report the dependency issue when the mod was updated. (it only appeared after I disabled the mod, exited the game, and when back to the screen).
4. The error in the save file was not clear to assist in the diagnosis - better error message would help.
Given how critical reliable saves are in a game that is about 'commitment' to your character, please investigate.