Okay, so deleting the hst file didn’t work at first, but that did give me an idea that eventually managed to get the game to save the options. Side effects: unlinks account. Exact steps:
- Unchecked “Keep games saves in the Steam Cloud for Chicken Invaders Universe” in Properties > General in Steam.
- Uninstalled the game
- Deleted folder 1510460 (Steam\userdata\User_ID\1510460)
- Deleted InterAction studios folder (ProgramData\InterAction studios)
- Made sure the Chicken Invaders Universe folder was deleted (Steam\steamapps\common\Chicken Invaders Universe)
- Reinstalled CIU.
- Re-link account
(forgot to add; thank you for the help)