I've gotten "object too deep" when saving but only when I tried to save and I was using an autosave setup. Basically I was trying to save, while my game was trying to save.
I'd argue checking any events or plugins you have that can control saving and test those out.
Made a brief video showcasing early game play... Got a copyright claim by BIG UP! (https://big-up.style/) for using music I bought from here (Future Steam Punk Pack). Now I wonder if I should strip any purchased music from here out of my game.