surprise. I got it vanilla. Well, almost. lol
What I did to make it pop up:
I created an entirely new save. Single sim, pet.
I used MCCC's debug cheats to make the pet run away (ok I admit I have been using MCCC for that. I don't have time rn to wait for the pet to run away on its own. Can do that tomorrow)
Anyway. Let the pet run away, Save&Exit. When I got back into that household, the pet was still missing, just as I wanted it to.
Now, my next suspect was that the time for that run-away-action has passed and the pet should have returned already (1-2-ish days I guess?) and that's what broke it.
So, off to the BFF household, let them play for two to three sim days, then save&exit again. booted up the game again and went to load up my first household and: tada, I present to you, a fresh Error Code 131.
I then go to load my household again, and, tada, a LastException is created. I attached that file here as well. Fresh one.
Pet is gray in that lil menu down there, but present on the lot. moves, lives. Also got the notification that dog has returned and I should greet her.