We faced the same error after upgrading Rancher to v2.11.2.

I am not 100% sure, but I believe running fleet as a StatefulSet is no longer expected. Instead it is always running as a Deployment, so that you can have multiple pods for failover situations.

We managed to fix this error by triggering a complete re-deploy of the downstream fleet agent: https://fleet.rancher.io/troubleshooting#agent-is-no-longer-registered

Afterwards the StatefulSet got removed, a new Deployment created and the fleet-agent Pod ist running without any errors. The cluster can now be viewed in the Continuous Delivery interface and is "Active".