Bad Handshake errors after EKS upgrade

After upgrading K8s version on EKS to ‘v1.27.12-eks-adc7111’, the cluster agent fails to enter a ready state on the local Rancher cluster.

Checking the Rancher server logs, I see these errors:

2024/05/08 08:23:26 [ERROR] Failed to connect to peer wss:// [local ID=]: websocket: bad handshake
2024/05/08 08:23:27 [ERROR] Failed to handle tunnel request from remote address response 400: cluster not found

On the EKS cluster, there are no obvious errors on the cattle-cluster-agent, nor the rancher-webhook

Any ideas what might be causing the 400 errors + bad handshake errors? Or advice on where to look? I’ve tried de-registering, removing the rancher-related namespaces and re-registering the EKS cluster, as this has helped in the past, but no luck there.

Rancher version: v2.8.3
Running on k3s version: v1.28.8+k3s1
EKS version: v1.27.12-eks-adc7111