The issue occurs because the game saves files directly to Documents/HAJQK, but Android has not granted it “All files access.” Without this permission, the game cannot access a valid save location, so saving and save backups fail.
You can try:
- Go to Settings → Apps → Special app access → All files access.
- Enable permission for House of Shinobi.
- Completely close and reopen the game.
It totally slipped my mind to leave a note there.