Elastic Beanstalk deploys failing with 403 error for CloudWatchMonitoringScripts-1.2.1.zip

Hi, am trying to deploy to two of our Elastic Beanstalk environments, but am seeing the same error message on both:

Command failed on instance. Return code: 1 Output: Failed to retrieve http://aws-cloudwatch.s3.amazonaws.com/downloads/CloudWatchMonitoringScripts-1.2.1.zip: HTTP Error 403 : <?xml version="1.0" encoding="UTF-8"?>. EBExtension failed. For more detail, check /var/log/eb-activity.log using console or EB CLI.

And am unable to download CloudWatchMonitoringScripts-1.2.1.zip in the browser:

<Error>
<Code>AccessDenied</Code>
<Message>Access Denied</Message>
<RequestId>SV3EXS944RMSWXYG</RequestId>
<HostId>EOZ/h8Ch09luQ89e75FfHItOwkBbkD6naSo7xDQC+bXCACM89oZzWjzoSsecDfditFOHd0mTcHTCxL+rSBLYiA==</HostId>
</Error>

Any thoughts on how to resolve this?

(Full disclosure: we're using the "Multi-container Docker running on 64bit Amazon Linux/2.11.1" platform, which is officially retired - could that be the problem? We have plans to upgrade, but really just need to get these updates onto the servers for now.)

Thanks!