S3 - Access-Control-Allow-Origin Header

S3 now expects the rules to be in Array Json format.

You can find this in s3 bucket -> Permissions then -> scroll below -> () Cross-origin resource sharing (CORS)

[
    {
        "AllowedHeaders": [
            "*"
        ],
        "AllowedMethods": [
            "GET",
            "HEAD"
        ],
        "AllowedOrigins": [
            "*"
        ],
        "ExposeHeaders": [],
        "MaxAgeSeconds": 3000
    }
]

Usually, all you need to do is to "Add CORS Configuration" in your bucket properties.

The <CORSConfiguration> comes with some default values. That's all I needed to solve your problem. Just click "Save" and try again to see if it worked. If it doesn't, you could also try the code below (from alxrb answer) which seems to have worked for most of the people.

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
    <CORSRule>
        <AllowedOrigin>*</AllowedOrigin>
        <AllowedMethod>GET</AllowedMethod>
        <AllowedMethod>HEAD</AllowedMethod>
        <MaxAgeSeconds>3000</MaxAgeSeconds>
        <AllowedHeader>Authorization</AllowedHeader>
    </CORSRule>
</CORSConfiguration> 

For further info, you can read this article on Editing Bucket Permission.