I had the same issue and my game was pretty much freshly installed. I had only played one family, a single sim that I had made, she had no pets. I had played her for about 30 minutes, then not touched the game until the Ranch expansion came out. I played a new family, one sim and a horse. No dogs or cats, no pets had ran away as far as I know. Then I tried to load the game the day after release and that's when I had the error. I followed your advice and loaded the other sim I had made, had no issues there, then transferred to my newer sim. Not had any problems since, and I've made a new family of one sim, a horse and a cat.
I have all expansions except for the first fits kit. One of the premade sim families may have had a pet run away, but I had only played the game for about 30 minutes after installing it, then played it for two hours on the ranch release day, so I'm not sure if a pet could have ran away in that time.