An application runs on a fleet of Amazon ECS instances and stores data in an Amazon S3 bucket. Until recently the application had been working well and then started to fail to upload objects to the S3 bucket. Server access logging has been enabled and 403 errors have been identified since the time of the fault. The ECS cluster has been setup according to best practices and no changes have been made to the S3 bucket policy or IAM roles used to access the bucket. What is the most LIKELY cause of the failure?