I've had this problem too, and what seems to cause it is if something else in the game has not loaded in, and you go to save. Example, if your sims avatar picture in the bottom left corner is blank, and you go to save, it will get stuck, because the game is trying to save something that isn't 100% loaded. The biggest cause for this is Custom Content hair, which often doesn't load properly in your Sims avatars, especially on Children and Toddlers. There is a similar glitch in CAS, where if you try to upload a household to the gallery while preview pictures on the right haven't loaded in, you will encounter the same problem. Hope this helped. 😉