I had this problem too, which was resolved by adding a couple of lines to the policy document:
"Resource": [
"arn:aws:s3:::your-uploads-bucket",
"arn:aws:s3:::your-uploads-bucket/*",
"arn:aws:s3:::your-backups-bucket",
"arn:aws:s3:::your-backups-bucket/*"
]
Should this be added to the OP (which isn’t a wiki)? Or perhaps in the OP of Set up file and image uploads to S3?