[RL] Save Error 423:<various numbers>/Issue with Secret

I bought the new Royalty and Legacy EP and ran into this save issue before on a lot in Ondarion. It turned out to be because Tybalt Capp died in my pool but was still alive in his own household. So I just went to a prior save and didn't let him drown this time.  But now it's happening with my heir while she's at college. I thought maybe it was because of the house in britechester she was sharing with her cousins, so I loaded a previous save and moved them all to Glimmerbrook. I was able to play for a bit but then eventually I still got the same error code when I went to save. I have repaired the game through Steam, where I play from. No mods, no CC. What else could be causing this?