Problem connecting cluster with certificate

After following the rather foolproof instructions [here] (Connect existing clusters through cluster certificates **(DEPRECATED)** | GitLab) I see the message:

There was a problem authenticating with your cluster. Please ensure your CA Certificate and Token are valid.

Which I have done numerous times. The odd thing is that my colleague added a different K8s cluster without the error. The one thing that comes to mind is that the working cluster is using a Rancher self signed certificate and the troublesome cluster is using a Letsencrypt certificate. Given the recent root certificate expiry issue that impacted Letsencrypt, could this be my problem?

Many thanks for any input…