For anyone landing here in 2025, where Keda is currently sitting at v2.17.0, I needed to add this to my serviceAccount.yaml after encountering similar problems:
eks.amazonaws.com/sts-regional-endpoints: "true"
So entire serviceAccount looks something like this:
apiVersion: v1
kind: ServiceAccount
metadata:
name: <SA>
namespace: my-namespace
annotations:
eks.amazonaws.com/role-arn: arn:aws:iam::<ACCOUNT_#>:role/<SA>
eks.amazonaws.com/sts-regional-endpoints: "true"