Hi Andreas!
I think you might be running into [Stack Monitoring] Missing remote_cluster_client role is reported as "access denied" · Issue #129546 · elastic/kibana · GitHub can you check that remote_cluster_client is added to the ES node being used for stack monitoring?
Alternatively you can set monitoring.ui.ccs.enabled: false in kibana.yml to avoid attempting to query remote clusters when loading the stack monitoring UI.