s3 Policy has invalid action - s3:ListAllMyBuckets

As zdev mentioned, you need to do this for the IAM. Go to the IAM console and navigate to Users > Permissions > Inline policies > Create > Custom, and enter this:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:ListAllMyBuckets"
            ],
            "Resource": [
                "arn:aws:s3:::*"
            ]
        }
    ]
}

I figured out myself. It needs to be done in the IAM, not in S3 itself...

@dnlbrky You need to do this by setting the policy on for the IAM user/group/role and set it by either using the AWS console for the IAM user/group or by calling put_[role/user/group]_policy boto API call.