Your application cannot read objects from S3 because the bucket policy incorrectly denies access. Fix the policy by allowing the correct IAM role and removing overly-restrictive conditions.
Your EC2 instance cannot reach the internet. The security group is correct, but the subnet route table does not map 0.0.0.0/0 to an Internet Gateway.
Your Lambda function started timing out after adding it to a VPC. This happens because Lambda placed in private subnets cannot access the internet or AWS APIs unless NAT Gateway is configured.
Your ECS Task or Lambda cannot assume its IAM role because the trust relationship is missing the sts:AssumeRole action.
Your S3-hosted website returns 403 errors. Public access is blocked by default. You must enable static hosting and add a correct bucket policy.