I believe I am on an Xbox 1 or Xbox 1S and I have 368.4 MB of Sims data on my account with 174 GB free on the console. Packs I downloaded most recently (between 3-6 months ago) were the Horse Ranch pack, Enchanted by Nature/Pre-Order Content, and two kits.
I have saves over 4 years old which I have avoided touching out of fear that this issue will delete them. This past year, I had played a little every once in a while and I only got error 34 (trying to save over an old save file and it deleted the old one) for the first time a couple weeks ago after months of not playing and I resolved the issue by creating a new save.
The game continued saving normally until today, where I have gotten error 34 consistently as well as error 21. I noticed error 34 occurs when I try to save over an old file, while error 21 occurs when I try to create a new one. My old saves are being deleted if I try to save over them and I cannot create any new files.
I have not experienced any issues with the gallery or saving Sims to my library, but I tried the advice someone gave about deleting families/lots in my library and it did not help. Autosave is broken.
I see that this issue goes back as far as six months, which is worrying to me as I have bought nearly every pack along with few kits. If there is no way to save the game, there is no reason to play and all these people experiencing this issue should receive refunds.